EDIT: Of course, I meant cease, and not seize....
My family has been in Quebec since the 1670s, and most of the French Quebec speakers had their family immigrate in the 1600s or at least before 1763.
All talks of immigration to Canada between 1763 and 1945 talk about British subjects moving to Canada, apart from some Chinese immigrants in British Columbia and a few other sources (such as American slaves fleeing to Canada to avoid slavery)
No one seems to talk about French people moving to Canada during that period.
I know that there was a regular influx of French Bishops from France named by the Vatican during at least a portion of that period: it was one of the concessions of James Murray (first British governor), which was codified in the Act of Quebec a few years later (1774).
But what about other French residents?
What about a peasant who was trying to go to the Nouvelle-France and was planned to go in 1764?
Was it illegal for them to come to Canada?
If it was, when did it stop being illegal, because there are immigrants from France right now in Canada.
I don't think it was illegal. The problem was that crossing the Atlantic to migrate to Québec post-1763 would be hard just for the fact that France and Britain were at war constantly. They could go and some famous French people did. Some include François René de Chateaubriand (Mémoires d'outre-tombe) and Alexis de Tocqueville (Regard sur le Bas-Canada). Tocqueville's account is particularly of interest since his trip was in around 1830. He was expecting to see little French. He wrote beautifully of the surviving French culture and lamented that France had changed so much. He said that New France was now in Europe.
The migration aspect of your question remains to be solved and I hope someone will answer it but I hope I shed some light on it.
Btw: my family has been in Quebec since the 1640s :)