Let me clarify some things here.
- I-485 if for future employment, so it doesn't require u to work for that employer currently.
- Filing I-485 gives u Adjustee status which allows u to continue to live in the US without working. U would need EAD [if u don't have H1 or some other status] to work.
- There is no such thing as u can move to another employer after 6 months of filing I-485. Basically u can move to another employer any time and use AC-21. But the caveat is if it less than 180 days of ur I-140 Approval, then employer can withdraw/cancel the I-140. This invalidates ur I-485 as its I-140 is cancelled.
- So now, if ur I-140 has already been approved and it has been 180 days, u can freely move to another employer. If not, u have to been on atleast good terms with the current employer so that they don't revoke ur I-140.
- If ur employer doesn't revoke ur I-140, but goes out of business, if USCIS comes to know of it, or they randomly choose to send u an Employment verification RFE, then u should be able to submit a letter from ur current employer [whose I-140 u used for approval] or from another employer [Using AC 21], that they would be hiring u full time, once ur 485 is approved, remember [EVL would be needed for ur future employment].
Atleast this is how it works legally and is the procedure. I am clarifying this because there is a lot of misconception and didn't want u to be misled.