You cannot use I-140 from the old employer to get the GC from your new employer (unless i485 was pending for 180 days or more). The new company has to go through new Perm/i-140 process, however, the priority date from your old i-140 is retained and used for new processing if you want to use i-140 from old employer. This would be useful only if there is few years of wait like STUPIDA said, but now its current for EB2 and just few months of lag for EB3. I don't see any benefit of trying to go through that process.