The new version 2.1 is almost fully backwards compatible.
The migration guide from 2.0 to 2.1 can be found at
book.cakephp.org/2.0/en/appendices/2-1-migration-guide.html
New stuff for the upgrade shell
I used this chance to enhance my version of the upgrade shell and added – besides some minor fixes – a command cake21. It should take care of all api standard updates for 2.1: git commit1 and 2
The Upgrade Plugin can be found here.
It currently updates Auth::allow() and layout stuff ($content_for_layout, $scripts_for_layout etc).
Feel free to add missing replacements.