For muscles, at least, which are not single cells but actually syncytia with multiple nucleuses, weight-bearing exercise produces an increase in the number of n

For muscles, at least, which are not single cells but actually syncytia with multiple nucleuses, weight-bearing exercise produces an increase in the number of nucleuses and thus a relatively permanent increase in the ability of the syncytia to synthesize protein. So retraining tends to be about double or even triple the speed of the original adaptation. This is one additional reason you should be extremely skeptical about "before and after" photos