You should be able to use the current upgrade package for CS2008.5 SP1 found on our downloads page (http://get.communityserver.com/download/cs20085up) to upgrade any version of CS from 2.1 on to the latest release of CS.
One thing to mention is we have seen some clients have issues with the upgrade process when going from CS2.1 if only using the CS2008.5 SP1 upgrade file. This does not happen to all of our users, and the issues that have prevented the process vary from situation to situation.
If for some reason you do encounter errors when applying the SQL scripts to upgrade your DB schema, it might be worth while to attempt upgrading from CS2.1 to CS2007, then to CS2007.1 and then using the CS2008.5 upgrade file to go the rest of the way. If you do have to go this route, you can find the install files for straight CS2007 at the following location: http://get.communityserver.com/download/cs2007web/