I think it's more than Clarke has been excellent at CB so far and why change two positions instead of one?
Willis will mature into a CB, nothing wrong with a stint at fullback, lots of good center halves have started like that (you might count Clarke among them soon). Phillips looks a more natural option to back up Christie, but again Willis is probably slightly further along his development.
Great to see the kids step up, we still need a bit more experience overall, but no-one who has had to come in has disappointed.