The new employer will need to start your GC process from scratch if you want to switch from EB3 to EB2 queue.
Restarting GC process means anywhere between 10 to 18 moths before you get the I-140 approved. If your PD is current in EB2 at the time your EB2 labor is approved, you may be able to file I-140 and 485 concurrently.
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