I am working on H1B for a very reputed company "A". "A" is an excellent employer and I have been a good employee. "A" filed for my GC and I have an approved I-140. I-485 not filed as dates won't become current for many years.

I am joining another Company "B" (H1B Transferred) in a few weeks. However I would like to request "A" to not revoke my I-140 so that I can possibly come back to them in the future if they want me to. Before I request that, I would like to make sure my request is fair. Hence:

1) Is it required by law for "A" to revoke my I-140?

2) If not required by law, what can "A" possibly lose by not revoking I-140? Is there any risk to "A" as dates won't be current for many years?

4) Is there an advantage to "A" in not revoking it?