Ok, so i'm replying to this half asleep...
From what I understand, your voice is getting horse:
1. You are over working it(in rehearsal), which we tend to do when we be feeling that song in rehearsal.
2. You are singing from your throat, which is gonna cause this to happen.
I recommend warming up/practicing every day. Your voice is realizing you are trying something different, singing higher, but it is not used to this. Just like over working your body muscles....you get tired. So take care of you voice by using it every day, singing high( but don't over do it) when you can, and keeping hydrated. Gotta keep them chords moist...refer back to the practice methods thread I did a couple of months ago. I'd love to provide the link, but i'm too tired. Hope I helped, if confusing...PM me.
Much love, many blessings...
Miss Klarity