The version we're targeting is 4.0 since it's the latest XNA version. Atm. we have no concrete plans about supporting older versions, which might become a future plan but currently we're busy getting 4.0 feature complete.
A good help for porting XNA 3.1 to 4.0 is this page: http://nelxon.com/resources/xna-3-1-to-xna-4-0-cheatsheet/
We're always happy to see people switching to ANX, so i hope you'll give it a try, porting your Engine :)
I'll post the targeted version on the front page right away!