You’re definitely overthinking this. IF such a deal was made, and that’s a massive if, the treaty would spell out exactly what happens to the people living on the land and those details would be points of negotiation.

Frankly the ideas of the other commenters that they would only be granted PR are ridiculous.

If we annex a chunk of land then we need accept the residents as our citizens. The finer details are an open question for treaty negotiations. I would guess there are requirements like it only applies to US citizens whose primary residence is within the territory at the time of signing the treaty, or at the time the handover occurs, or something in between. Maybe the residents have a choice: exchange American citizenship for Canadian, or keep American and get Canadian PR.

These questions are easily solvable but exactly how they’re solved is up to Ottawa and Washington. I doubt the US would ever give this land to Canada though, they will want something in exchange.