"job Description" would play rolw even after your i-140 is approved, AC21 law says that new job must be under same category, donno if it has something to do with job description or there are certain job codes under which we file our applications, but it sure that changing employers on EAD/AC21 is bit unsafe before I-140 is approved.
As far as RFE's concern, I guess, on I-140 level, there are possible queries about sponser,s o he would have to reply for them, but on i-485 level i guess queries are specific to you and not the sponsor so you will have to reply them.
hope this helps.
-manoj