Some of the Escadrille Lafayette US pilots stayed in the French service throughout the war. Some transferred to the USAS like Lufberry but only once the US had joined the war. I think with the RFC pilots it was somewhat different because to get around the rules of neutrality they were 'officially' Canadians though they never tried to hide the fact that they weren't. Example is Bill Lambert DFC of 24 RFC who was 'demobbed' from the RAF in 1919. He joined up in Canada and was trained there. This most likely saved his life as the training in England was useless compared to that given in Canada. This explains why so many Canadians became aces and so many English pilots were cannon fodder.
The US pilots on the French side joined the Foreign Legion first before becoming pilots so were 'officially' French.

Note that in WWII when the training was good on both sides of the Atlantic the Canadians did well, but not so overwhelmingly well as they did in the First War. And then many British pilots were trained in Canada and the USA (and other places like South Africa with better weather for training).

Catknight - after being taken out by my own wingman (never saw him) and lucky to crash wounded I avoid my own flight like the plague! Normally 500ft higher and off to one side so I can keep an eye on them