The beautiful 49th parallel border is probably largely gone- OTL the only permanent cession of American territory of which I am aware was the straightening out of that border, with American claims North and British claims South of the parallel both renounced. TTL there's no incentive to do that- the American's claims North of the parallel will simply remain with the states concerned- I believe it was mostly the Dakota territory, but I could be mistaken. Rupert's Land is probably too big to govern efficiently as a single entity, so it will probably get broken up; but all of the political boundaries inside the territory post-date 1815 so the borders will probably be very different- and largely dependent on the needs of the political parties at the time the borders are drawn.
One factor there: how does the Hudson's Bay Company make out, in an ATL where their fur trade monopoly is presumably dead as a doornail? Without the HBC there's a huge power vacuum in Rupert's Land, and somebody's going to have to fill it...
Upper Canada and Lower Canada, not to mention Nova Scotia and New Brunswick, already exist as political entities. Changing their borders arbitrarily probably doesn't make much sense, although there's no guarantee that Upper Canada will get the huge chunk of Rupert's Land that became Northern Ontario or that Lower Canada will get quite so much in its border dispute with Labrador. To put it another way, a map of TTL's Upper and Lower Canada may look very different from OTL, but the major population centers are probably roughly in the same place (except Ottawa, which is more or less gone) and probably belong to the same entity as OTL. PEI is the odd man out, already a separate colony but somewhat likely to get folded back in to Nova Scotia simply because its kind of small- although as Rhode Island can attest, that's not necessarily an insuperable obstacle to statehood.
And now for the elephant in the room: slavery. All these potential states, which Americans- including many Southerners- have fought and died to "liberate" (meaning giving them up is unacceptable), are likely to become Free States if admitted to the Union. (Although Upper Canada's full emancipation process had not yet entirely finished, and the rest of the colonies did not ban slavery- but in spite of being technically legal it was vanishingly rare and increasingly unpopular.) This means that some members of the House of Representatives and Senate have an incentive to make any states admitted to the Union as large as possible to try to minimize their political power, and to delay their admittance as much as possible. This might have big effects on how alt-Canada ends up; but such effects are easily butterflied.
Does the US accidentally converge on Lord Durham's solution and combine Upper and Lower Canada (with at least all the problems that caused OTL, one would think)? Do they unite PEI, Nova Scotia and New Brunswick into one state (essentially similar to OTL's proposed Maritime Union, but part of the USA)? There's lots of possibilities there, but they're too hard for me to predict.
