First of all please look at the BREAKING_CHANGES:
We add new items as far as we find new breaking changes.
The Examples may be helpful as well:
Also, a lot of migration problems have been answered on the Premium Help 2.x and Help 2.x. forums. So, please use forums searching.
If you won't find an answer, please ask on the Premium Help 2.x starting a new thread.
Here is a forum thread to post about breaking changes that we missed:
Also I think ExtJS migration helpers can be very useful, because many of our breaking changes came from ExtJS.
EDIT: Just to be comprehensive, there are some concerns from the community regarding migration from ExtJS 3 to 4, so, form Ext.NET v1 to v2.