Need to Join previous employer after EAD approval?

[H1BUser24]

I switched to a new employer couple of months ago who has not initiated my PERM process yet, they may start PERM process some time in 2021. I had my I-140 approved through my previous employer with priority date 2012. I see the EB2 priority date advanced to May-2011 and EB3 to 2015. I would like to know if I could ask my previous employer to downgrade in EB3 and file I-140/I-485 (if they agrees). Let's say, they agrees and I get EAD approved in 6-8 months, would I have to join the previous employer after EAD approval?

Thank you.