I understand, but the thing is, the mechanisms involved with an accent change are the same regardless of the reason.
Just my own experience, but how I speak and pronounce a given word, isn’t through hearing myself moment by moment, and choosing how to say it. It’s more like launching an ap on a computer. I find the place in my mind where the word it stored, and “click on the icon” to launch it. And where I have different accents, I have entire libraries of word pronunciations stored together, so that once I start speaking, I will remain with that accent until I finish the sentence. Shifting from one accent to another mid sentence requires conscious effort.
I suspect that some people retain a memory of how to contort their mouths and throats without changes, more than others do. Hence some people revert more easily than others. But I suspect the most common result, is that even people who dorevert to an older accent, never quite get back entirely to it, because over time, if they remain using a desired accent long enouugh, their memories are replaced word by word.
Plus, any word that we learn anew in an acquired accent, will only be known IN that accent, so even if most of our “foreign” accent disappears when we go back home, any new words we learned will still be spoken with the acquired accent. Here in DC, I deal with people from all over the world on a daily basis, and I’ve long noticed that no matter how strong their home accent is, any words that they did not learn at home in English before they came, will be pronounced with an American accent. It’s really quite funny.
Most amusing of all, are the surprising hybrids: I ran across a guy speaking with a strong Jamaican accent a while back, and since I didn’t think he looked at all Jamaican, I asked where he was from originally. he answered “Russia” with a VERY strong Russian accent. But he had learned all his English from a Jamaican, so that’s how he spoke it, including saying “mon” at the end of every other sentence.