Transposing or transcribing?
In other news, here is your "El-Paso-Culture-In-A-Nutshell" comment for the evening:
"You're 37? How you a 37-year old man and no kids?"
Transposing.
When you're on the worship team you have to do that a lot.
Someone ends up saying something like, "The key of C doesn't work, we'd be better off doing this in F#!" and then the fun begins! It only takes me like 10 seconds because I've been transposing songs since I was 13 but the rest of the team has to catch on.
It's so easy though. Most songs only use the 1sts, 4ths, 5ths, and (minor) 6ths (like if it's in the key of G - most worship songs just use G maj., C maj., D maj., E min.) of keys anyhow (and surprisingly the majority of the time IN THAT ORDER). So you just gotta know what those are for each key and the rest kind of fills in the blank for itself...but if you know your major scales then it's a cake walk.
But nobody else actually understands that, so when we change the key to a song, I usually am the one to tell the bassist what notes most likely need to be played, and tell the pianist what the chord progression is.