Switching to MAC, need help

I can't comment on running PA9 under Boot Camp or Parallels.

BUN files are proprietary to Cakewalk and I'm pretty sure that no other program can open them.
I once moved from a Mac to a PC and had to use a brute force method that worked but wasn't much fun.  Based on my experience here's what I think you will have to do.

In PA9, open the BUN file and then export each audio track to a wav file.  Then export the midi tracks as a SMF (Standard Midi File).  Move the files to the Mac, open the SMF and import the wav files.  (I'm assuming that the Mac can import wav files or there is a utility that will convert to something the Mac likes.) 

This will get you back much, but not all, of the stuff you had in the PA9 project.  Some midi info can be lost because PA9 supports midi features that are a superset of the SMF standard.  There will also be issues with any real-time effects you used in PA9.  The exact same effects probably will not exist in the new environment so you will have to re-do them as closely as possible using the new effects.  I suppose you could apply the audio effects to the audio tracks in PA9 before you export them but then they are permanently in the audio and can't be changed.  If you have PA9 audio clips that do not start at the beginning of the track, you will have to manually move them to the appropriate times in the Mac environment.

Such a brute force approach is not any fun and can take a lot of time.  And then you don't end up with exactly what you started with.

Maybe you will get lucky and be able to run PA9 in Boot Camp or Parallels.  PA9 is not a big cpu hog so it might work.  I'd think that Sonar would require more resources.

--Larry