I don't know if a part-time job can become the replacement job under AC21, especially if the original job was full time (but maybe it can ... see a lawyer if you need a definite answer).
But since there is no actual requirement to work for the sponsor (or anybody) full time before GC approval, it should be OK to work part time in anything as long as you still have an offer to work full time in a "same or similar" job after GC approval. And you should actually go to work in it full time once your GC is approved, in order to avoid problems during naturalization.