I-140 thru another employer

[greatdesi_2]

Hi,
I am with Employer A.But filed a PERM thru Employer B as a future employment basis.Recently my PERM (thru Employer B) has been approved.My questions are

Can I file I-140 immediately or any wait time to file it thru Employer B?
Employer B is flexible on transferring my visa.Is it advisable to transfer the visa after the approval of I-140? or after getting EAD?

Thanks in advance!
Desi friend