First make sure that you save copies of your PERM, I-485 receipt notice (i.e., Willy Wonka Golden Ticket) and I-140 (optional). Also get the copies your I-765/I-131 for later use. The PERM (SOC code and job description) will be of great help when you do AC21.
Count 180 days from received date and then initiate your H1B transfer. While getting the job offer / negotiations tell your future employer that they also need to do a AC21 / Supp J while doing the H1B transfer. My guess is you will get your EAD/AP within the 180 day period. Even if you transfer, EAD/AP is provided on the basis of a Pending I485. As long as I485 is pending you will get your EAD/AP. But if they are doing H1 transfer then no harm in staying with H1 status.
- Initial EAD/AP typically has a validity period of only 1 year. You can renew it 180 days in advance. Assuming that you get your EAD/AP around the 150+ days mark, then the new employer cannot apply for renewal immediately. You can do it yourself following the guidelines in our EAD/AP forum.
- If the expiry of the EAD/AP is within 180 days, then the new employer can file. I485 Supp J will get its own receipt number and approval. It helps to have a copy of your PERM.
Good Luck.