On the subject off adding/replacing tiles: for a small game (base 72 tiles), I can see it making sense to replace the old cloisters. When adding other expansions, there will inevitably be more cloisters, so they can't all be replaced. I think I would definitely replace some of the original cloisters if playing with these, but it would depend on which other expansions I was playing with.
I agree - given the number of expansions we have, and how hard it is to actually complete a monastery given the dragon and the tower, just adding the expansion makes sense.