Like SJ said, it mainly a church to church and to me it's also a maturity thing.
In my church, if the M-O-M says we are singing "such & such" and the leader says I'm not gonna be here, then M-O-M says well plan B, "so & so" you know this song don't you? "yes ma'am" and she says "well lets go", and the regular leader is cool wit it. But we do have some people that will get ticked off if the choir is singing "their song" and that person isn't gonna be there. The M-O-M is like oh well, won't be the last time we gone sing it.
Same thing with musicians, directors, & choirs. There are some songs that the youth choir sing that the adult choir does, and some that the adult choir does that the Senior Choir usually does. SO WHAT, IT'S JUST A SONG!