I am sorry, I did not make that clear.
My use cases are as follows.
(1) For performance, I use the song in Biab display mode (not playing Biab in playback mode), as just a lead sheet with just chords and lyrics, and I already know that I can change the key at-will easily so when the singer says "hey let's try this in the key of C major so everybody transpose now" I can just click and transpose then and there.
(2) For practice, I will set the key and then playback in Biab (with Biab accompaniment) as usual.