I am working on a medley of Carols for a special program and dont have much time left ( like 2 weeks) and we have to rehearse them too. I would like to take some carols like Hark the Herald Angels sing, Angels we have heard on High, and Joy to the World, and combine them somehow with Jazzier updated chords... Would appreciate suggestions, but not necessarily someone to do the work for me. Is there a basic theory idea or rule of thumb behind chord substitution that will help me change it up a bit? I am also working on another idea for some old old songs... Thanks.