Was FMC >6ft Ever an Acceptable Grounding Path?
Whether or not you are &quot;grandfathered&quot; it is really foolish to leave a garage door opener on an ungrounded circuit or weakly grounded circuit. (which is what FMC is - it's a weak ground)
People have been killed by something going wrong with their ungrounded garage door opener, they get a ladder out to climb up and take a look at it, touch the unit or wires in it, get a shock and fall off the ladder.  It's not the shock it's the fall that gets 'em.