Thank you all for your answers, much appreciated

@Matt yes, I also hope that PG Music will add this feature in the future. After all, the advantage of the multi-riff option, which is truly fantastic, is limited by the fact that by freezing that perfect track of yours you're also committing (unwillingly) to that key. Which is a pity, as it takes away from the great flexibility of BIAB. And, as you said, since frozen tracks are not saved with the song as full audio tracks, but only the instructions are saved to play back the frozen track as those instructions dictate, then it should be relatively easy (I guess ...) to allow frozen tracks to be key shifted while not losing their "RealTrack status".

@musocity yes, just like you, I also work on a separate DAW. Transposing tracks in a DAW is not an issue. My question was about how to do that in BIAB with frozen tracks, and that is relevant (for me, at least, maybe not for others) because I really love the multi-riff function of BIAB, which naturally leads to freezing tracks, obviously. But, if we want to transpose a frozen track, then we lose the multi-riff / regenerating flexibility, and if we want to keep the multi-riff flexibility, we can't transpose. Unless there's another way that escapes me.

Anyway, as Matt suggested, this is one for the wishlist.