You can always have performers sing things in a lower octave if they can hit the notes. Or even just the parts of the songs that are too high.
If you want to bring the songs into a different Key you run into a few different issues, because it's going to affect the other performers as well and the instruments playing the music.
if you are using Orchestra for the performance, as you noted, it can change the keys. Rehearsescore also allows you to alter the key, tempo, make cuts etc. This is the best option. I don't know if Rehearsescore is available for HSM, but if it is it's worth the investment.if you are using a track, you could lower the key by using a program like Adobe audition or Apple Logic, but again, you're going to have some songs that sound weird because some instruments can't play in some lower keys.
My strongest advice is to have the actors sing the songs in the original keys and alter the sections of the song they can't hit through different voicings of just those parts. That way it sounds like what everyone is used to hearing except for the trouble areas.
I hope this helps.
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