I try to only choose songs that fit the group. I know their strengths and weaknesses, and while I try to work on their weaknesses, there are some songs, and some notes that they just shouldn't sing. So yeah, I may alter a song by a half-step or so, or I may alter their individual parts, if i deem that its too difficult for them to catch on.
I try not to change songs too much (like dropping 2 or 3 keys, or leaving out half the song)....it wont be the same. So it just kinda depends on the group, and the song that you sing.