I am in a similar situation.I was told that if your previous employer has to keep your labor intact without selling/use it to anyone.The point is, you have to have file another Green Card through Perm through your new employer, file your I-140 and then use your previous Labor priority date to file your I-1485.As you know, green card is for future employment.Anyway this is the info i got,please let me know if you find any info.It will be of great help for me.
Regards,
GC Prospect