You can change your employer with your approved i140 and still port the EB1 priority date. This is allowed.
But, the only caveat is that a new i140 approval would be required to be filed by new employer. New i140 is needed if you want to file your GC (i485) as and when it gets current.
If you plan to return back to old employer with approved i140 at the time of GC filing, you can avoid filing i140 with new employer.