It's possible to change FF completely, but I don't think it makes much sense in light of their TOH differentiator*, J2 can still have compelling specs without changing FF dramatically**.
If they don't have anything more interesting on the horizon for their TOH story, then it'd make sense that they change FF dramatically, but rumours we've heard suggest otherwise.
If there's finally some compelling stuff coming to TOH then J2 should be the same FF, as the TOH ecosystem gets stronger they can think about migrating from current FF to 5"+ for the J3 or beyond.

*which they made a big deal about making them "innovative/unique" & "needing" (as they can't compete on the raw specs race), but which they haven't fully delivered on -yet
**for the $ I mean, especially once you add a compelling TOH story