I remember reading somewhere here that you should never mix pastes? Why is that? I suppose I could go and buy some balsa wood, but I've got a piece right now that has crox on it. What's the harm in putting some diamond paste on top of the crox?
Your strop will take on the properties of the coarsest paste. 3u paste put on a strop that had 1u paste before will be a 3u strop from then on. You cannot get it back to 1u. So, you can repaste a strop to make it coarser, but not finer.