PERM to be reinitiated if return to previous employer with whom have approved I-140?

Approved (and not revoked) I-140 with previous employer A and currently with employer B.

  1.   If I join previous employer (A) -

a. Does the PERM process have to be restarted?

b. Any risk in transfer back etc?

  1.   If I remain with employer B, can I still file for 485 when priority date becomes current? Do I have to join Employer A back for that?

Thanks in advance for response/suggestions.