You can start working with a different professor if you wish. However, in that case you have to redo CSE 523 if you still want to do the Advanced Project Option. Generally speaking, you cannot do CSE 523 and CSE 524 with two different professors as they are meant to be on the same general two-semester long project. The other option is to drop down to the Basic Project Option and take a CSE 522. In either case, your CSE 523 with Professor X will not be counted as a part of the 31 credits required for graduation.