Hi,
I think the laws re this probably vary from state to state. I am no expert but my understanding is that if you left a full time job and choose to come back part time, they don't have to give you your old job back because it is a full time job and you did not want to come back full time. Hope that makes sense. Nut then again if you have it in writing that you could come back to exactly your job part time, that would probably change things. No reason though why you should not be able to renegotiate your pay rate at any time.