We have a statement from @nrp that states that they plan to make future mainboards in the same form factor.
It’s a little vague, so there is a possibility that 12th gen mainboards could be a bigger form factor and other upgrades would be sized the same as the 11th gen boards, but since we know that Framework’s mission is to make sustainable computers, I would hope they would try to make all their mainboards compatible in all their laptops.