Hello!
I have a Gen Bb. I’m wondering if it’s possible that I loosen the mouthpiece and pull/slide it a little bit higher so that the notes would fall in to A, then it would be a Generation A whistle. Is it possible that the Bb be like that or sort of? thanks…
You can make it tunable by loosening the head in hot water … and melting the glue enough to remove it. Look up other threads for instructions on how to do this without damaging it.
I don’t think you can make it that tunable without making it out of tune with itself. Many whistles come in Bb/A combos in which the same head comes with different tubes for those keys. If it could be done just as well with a tuning slide it would be. Also, note that those combo whistles often aren’t equally good. If an A tube has been designed to go with a head originally designed for a Bb body, you might find the A whistle less good than the Bb.
I have tunable Dixon A and G whistles. I can pull the slide out far enough to get Ab and Gb, but it takes a bit of creative breath control to blow it into tune. Also, you have to pull out the head quite a distance, and I don’t think you can pull the head of a Generation Bb that far.
Why not get yourself a Sindt ‘A’? 
The more you change the tuing of the whislte the more it will go out of tune internally - the individual notes will not be in tune with eachother.
Chris