In theory, source code from SDK 1.x (A0) should compile and run without any issues on SDK 2.x hardware (20736/37) and firmware (A1),
However, as several users have discovered, there are small undocumented nuances which will require testing on your side to confirm the application works properly. Many of these nuances you will find are noted here on the forum and several have existed as bugs in SDK 1.x that users leveraged as part of a custom feature. In this instance, once SDK 2.x fixed the bug, the custom feature went away and they had to figure out a new method to accomplish the same task.
Here is one such example of an issue a customer ran into when moving from SDK 1.1 to SDK 2.x
The good news is that many have made the transition successfully.
The online HTML documentation has been updated in SDK 2.x, but where you will find the most improvements is via the sample applications that are provided (many of them contain documentation inline).