If this band has handwritten charts, there's really not anything out there that can handle those. But I've used this free tool with mixed results. Still requires a lot of manual re-work in Musescore after the fact, enough in most cases to make it useless.
musescore.com
I'm guessing a guitar band is going to have pretty simple horn charts that would be quick and easy to manually input into Musescore and the click the transpose button. Similar story for the free Cakewalk software. Or go old school and simply re-write by hand, transposing as you go. That's usually about as quick as entering into some program. Another option is just write flats in front of all the notes on a copy of the original chart. Lastly, sight transpose of course. That can work great on simple tunes. If you just have a few hits, simple lines and long notes, it should be pretty easy.
It wouldn't hurt to ask if they have digital copies of their charts already. That way it's just one click to transpose them. I imagine most composition is done digitally in the first place these days.
It doesn't make sense that they'd drop ALL their tunes a half step for the singer's sake. That kind of implies ALL were at the top of the singer's range to begin with, which seems pretty unlikely. But I'm kind of with you on just doing what they ask. If I pulled in a sub who asked me to change all my charts for his sake, I'd get a different sub.
Please let us know what you end up doing and how the gig goes.
