We Will All Go Together is the only example I can think of offhand.

Though the Sheffield Folk Choral do a piece called Yan Tan Tethera, which starts in D, then goes to Em, then goes to F#m, then goes to A, then goes to Bb, then...

I can't remember all the changes, Graham wrote it so that we had to change key every so often seeing as us folkies don't tend to have to.