I found the transition from Blitz Basic to Max pretty hard, but if you spend time getting to know it - who knows you might like it just as much.
I found it pretty easy as far as the procedural programming is concerned... But it does take time to wrap your brain around the (entirely optional) Object Oriented approach.
All in all, just the 2D-in-3D which gives you fast real-time rotating, scaling and transparancy is well worth it. I'd hate to go back to BB and give up those features now. :-?
Anyway: Even though there is an option in the BlitzMax IDE to import old BB code, it isn't of much use. Very few sources would work out-of-the-box after conversion, and for the most part it seems to take less time to re-write your old code from scratch than it is to try to automatically convert it and then have to tweak it to get it to work.