>> Originally learning all these movements probably took years of training and you still knew them so well that you could retrain your muscles to use them like back then.
Nah. I am actually making the opposite point: I had to consciously re-remember (from whatever I remembered my instructor told me) how it were back then and retrain myself to do it correctly all over again.
For example, getting over anxiety that I would be out of breath the next time I am breathing in. The only reason I could get over it was because I have consciously remembered I could do that when I was a kid, so it was just a matter of training and habit both of which I had to consciously push myself to acquire again.
"a few weeks" would only be related to "conscious remembering how it needs to be done", rather than "swimming like the old days". It took me a few months; constant attention and reading how to swim properly otherwise.