I am currently on GC EAD, PD 0/2014. If I change job, is my new employer required to file I-485 J before I join them or can it be submitted if RFE was issued or an interview is called by USCIS? Thanks for your time.

There is no rule that the I-485J has to be submitted at a certain time but it would be prudent to submit this sooner rather than later so your GC gets approved with the correct employer. Not working for the petitioning employer after getting the GC could raise questions during your citizenship interview.