Interesting - I can see a couple of equally-valid but very different ways to handle this.
One is the existing “Force into Scale” feature, as @Bernd described above. Depending on the two scales in question, this may change the sound of the chord progression quite a bit. The results might be great or not-so-great…
The other thing is actually what I expected “Maintain Current Tonic” to do, but it doesn’t. I expected it to auto-transpose the selected chord progression to the selected tonic (just like using the ST: button to transpose it by the right number of semitones, but automatic as you switch songs/artists). What it actually does is leave the sound of the chord progression unchanged but present the chords “in terms of” the other scale. (E maj is still E maj, but now it’s the V chord instead of the I chord.)
A good example is the Alternative 1 (A min) and Alternative 2 (E maj) songs. First listen to both song progressions to hear how they sound normally. Now go to Alternative 1 and pick one of the “lock” modes, then switch to Alternative 2. If you used “Maintain Current Tonic”, Alternative 2 will sound exactly the same as it did when the “lock” was OFF, but all the chord labels will change. Now transpose it down 7 semitones to A major and listen - this is what I expected “Maintain Current Tonic” to do.
Maybe it would make sense to have another option in the “lock” menu that does this sort of auto-transposition. I know I would find it useful!