Its a good question, as most of the studies I’ve seen investigate the effects of increasing in altitude, and not dropping down to lower elevations.
The studies I’ve seen on effects of altitude show a decrease in VO2max of 6-7% for every 1000m of elevation gain (and all else equal, similar effect on FTP), and that you can recover approx 1/3 to 1/2 of the loss with multiple weeks of acclimatization.
So let’s say you move from sea level to 1000m (3300 ft), your FTP will drop from 100% to about 93%. But then after multiple weeks you may acclimatize and recover to, say, 96%.
So if you go back to sea level, will your FTP return to 100%, or 103%? I don’t know, but my guess is somewhere in between. Maybe you can answer the question for yourself by doing an FTP test at lower elevations?