Alexander Limi has an excellent HowTo written for migrating from Plone 2.0 to Plone 2.1.  Here's the quick and dirty of it all.

As a rule of thumb, always start at the top with upgrades, and work your way down — upgrade Products, then Plone, then Zope, then Python.

Do the actual migration on a imitated test instance first — don't bring down your live instance before you know it works properly for your site.  Note: iold, and inew refer to the relevant zope instance versions.

Upgrade Your Products
This is most easily done by checking the version # in the ZMI at Control_Panel/Products.  For a list of ZopeWorld installed products please refer to the Plone Products Matrix.

Delete any objects

Uninstall in QuickInstaller

rm -rf ~/iold/Products/TheProduct
mv ~/backmeup/Products/TheProduct ~/backmeup/extraProducts

restart Zope

After you're finished upgrading your products, it wouldn't hurt to backup your Data.fs.

Upgrade Plone

cd ~/iold/Products
mv Archetypes ATContentTypes BTreeFolder2 CMFActionIcons CMFCalendar CMFCore CMFDefault CMFFormController CMFPlone CMFQuickInstallerTool CMFTopic DCWorkflow ExternalEditor Formulator generator GroupUserFolder MimetypesRegistry PlacelessTranslationService PloneErrorReporting PortalTransforms validation ~/backmeup/extraProducts/Plone-2.0.4
cd ~/pl252rc1
mv Archetypes ATContentTypes ATReferenceBrowserWidget CacheFu CMFActionIcons CMFCalendar CMFCore CMFDefault CMFDynamicViewFTI CMFFormController CMFPlacefulWorkflow CMFPlone CMFQuickInstallerTool CMFSetup CMFTopic CMFUid DCWorkflow ExtendedPathIndex ExternalEditor GenericSetup GroupUserFolder kupu Marshall MimetypesRegistry PasswordResetTool PlacelessTranslationService PloneErrorReporting PloneLanguageTool PlonePAS PloneTestCase PloneTranslations PluggableAuthService PluginRegistry PortalTransforms ResourceRegistries SecureMailHost statusmessages validation ~/iold/Products
~/iold/bin/zopectl stop
~/iold/bin/runzope
~/iold/bin/zopect start

Before you start the migration, you should decide what approach you want to use. There are two common ways of migrating:

1. Migrating your site content, products and customizations in-place.
2. Exporting your content, creating a fresh Plone 2.1 site, importing your content.

The in-place migration is more comprehensive, and hence more error-prone, especially if you have misbehaving third-party products or very old Plone instances. If your content is the most important thing for you, and you don't mind applying your configuration settings and simple customizations again, exporting all your content folders followed by an import into a clean instance might be a better approach for you. This procedure is described in the importing Plone 2.0 content into 2.1 FAQ. Please note that this should only be done if you are experiencing problems and as a last resort (or simply want to start with a clean site but keep your content) — for most people, in-place migration is the way to go.

1. Log in to the ZMI as a Manager user.
2. Go to portal_migration
3. click the migrate tab
4. select "dry run" and then click upgrade
5. if the process completes successfully, return and do the migration for real this time

Common problems

• If you have an item with the short name events or news in the root of your site, they should be renamed before starting the migration - since this can cause problems with the migration to the new Smart Folders that list these
• One of the problems that people run into during migration is third-party products they have installed that didn't clean up after themselves, or that left behind "dead" content when uninstalled. This can trip up the migration process. Here is a simple script that can list content with no associated product, so you can remove the defunct objects. To use it, create a Script (Python) from the ZMI add menu in the root of your Plone site, paste in the code from this file, click Save and then click the Test tab to run the script. It should list dead object locations, so you can go and delete them manually if needed.
• Another error that was often encountered in Plone 2.1 and 2.1.1 was that some objects weren't converted to the new Archetypes-based types. If you get: "maximum recursion depth exceeded" on viewing your site after the migration, the folders/objects are most likely still CMF objects, not Archetypes objects. Plone 2.1.2 includes a fix that tries to work around this problem. (The reason this exists in the first place seems to be bad behaviour introduced in the Plone 2.0 Release Candidates and subsequently fixed before the 2.0 final release, but some people still have content created with the Release Candidates.) Also note that this error message can show up if you customized a 2.0 document_view template and are trying to use it with Plone 2.1

goto portal_atct
goto type migraitons
click "Fix Types"
then click "Migrate"

After migration

• Several "new" tabs will appear at the top of the site. This is due to a policy change in how Plone constructs navigation. Plone 2.1 and up will automatically make tabs from the folders in the root, and doesn't require you to manually create them in the portal_actions tool anymore. To fix this, you can either:
• Go in and delete your portal_actions entries to only use the root folders (the folders also have individual visibility settings in the Properties tab of each item). This is the recommended approach unless you have global tabs leading deep inside the site, or:
• Turn off the automatic tab generation in the Site Setup → Navigation Settings. This will make the global tabs behave the way they did in 2.0.
• All the content items and folders that you have the permissions to view now show up in the nav tree - if you want the old behavior from Plone 2.0 back, where only folders show up — and only those who are published — you can now control the navigation setup in Site Setup → Navigation Settings.

Upgrade Zope

OLD/bin/mkzopeinstance.py

Directory: /home/USER/zope/instancenew
Initial User: USERNAME/PASSWORD

Now stop the old Zope Instance

instanceold/bin/zopectl stop

Remember to always backup your Products and Data.fs before you begin.

cp instanceold/Products instanceold/var/Data.fs backmeup

Copy over your old Products and Extensions

cp -rf instanceold/Products instanceold/Extensions instancenew

Backup the original working Data.fs for your new instance and replace with your old Data.fs.

cp instancenew/var/Data.fs instancenew/var/Data.fs.orig
cp instanceold/var/Data.fs instancenew/var

Restart both Zope instances

instanceold/bin/zopectl start
instancenew/bin/zopectl start

If you get the error AttributeError: _length, when you are upgrading to Zope 2.8, follow this HowTo by Lucas Zdych.  You will also need to call manage_convertIndexes on all catalogs that are not in the root (CMFCollector catalogs etc). Third-party products sometimes have their own catalogs, check with the product maintainer about this. See the section "Upgrading from Earlier Versions of Zope" in the file Zope-2.8.4-final/doc/FAQ.txt.

If you get the error: "Unauthorized: The container has no security assertions. Access to 'getInline' of (Products.ResourceRegistries.tools.JSRegistry.JavaScript object at 0x047F6130) denied."  Then uninstall VerboseSecurity.  In Zope 2.8.x, you can enable a verbose security option in zope.conf and therefore it no longer needs to be installed as a product.

Upgrade Python

How To Install Plone Products

 

You may wish to install some useful Plone Products to help power your website. You can find a full list of these http://plone.org/products as well as http://www.contentmanagementsoftware.info.

These products are installed in your Zope instance much in the same way Plone and it's pre-packaged products were installed. Let's walk through one, to be sure. Make sure you're back in your Products directory.

wget http://plone.org/products/atphoto/releases/1.0-alpha1/atphoto-1-0-alpha1-tar.gz
tar xzvf atphoto-1-0-alpha1.tar.gz

Restart Zope via ZMI/Control_Panel and verifiy that ATPhoto appears. To install products in your Plone instance use the QuickInstallerTool. Go to your Plone root, select portal_quickinstaller, and install ATPhoto. Success!

How To Install Archetypes

 

wget http://plone.org/products/archetypes/releases/1.4.2/Archetypes-1.4.2-final-Bundle.tar.gz

mkdir AT142Bundle

mv Archetypes-1.4.2-final-Bundle.tar.gz AT142Bundle

cd AT142Bundle

tar xzvf Archetypes-1.4.2-final-Bundle.tar.gz

cp Archetypes Marshall MimetypesRegistry PortalTransforms validation /home/USER/i296/Products

How To Install PyXML

 

cd ~/py243/lib/python2.4/site-packages

wget http://superb-east.dl.sourceforge.net/sourceforge/pyxml/PyXML-0.8.4.tar.gz

tar xzvf PyXML-0.8.4.tar.gz

~/py243/bin/python setup.py build

~/py243/bin/python setup.py install

How To Install MySQL-python

 

cd ~/py243/lib/python2.4/site-packages

wget http://easynews.dl.sourceforge.net/sourceforge/mysql-python/MySQL-python-1.2.1_p2.tar.gz

tar xzvf MySQL-python-1.2.1_p2.tar.gz

cd MySQL-python-1.2.1_p2

~/py243/bin/python setup.py build

~/py243/bin/python setup.py install

How To Install Trac

 

wget http://ftp.edgewall.com/pub/trac/trac-0.10.3.tar.gz

tar xzvf trac-0.10.3.tar.gz

cd trac-0.10.3

python ./setup.py install  --prefix=/home/USER/t0103

How To Install libxml2

 

cd ~/py243/lib/python2.4/site-packages

wget ftp://xmlsoft.org/libxml2/libxml2-2.6.27.tar.gz

tar xzvf libxml2-2.6.27.tar.gz

./configure --prefix=/home/zope/py243/lib/python2.4/site-packages/libxml2 --with-python=~/py243/bin/python

make

make install

 

How To Install Django

 

wget http://www.djangoproject.com/download/0.95/tarball/

tar xzvf Django-0.95.tar.gz

mv Django-0.9.5 dj095

cd dj095

sudo python setup.py install