The state supreme basically have free reign on how they set up their governmnet at this point right?
If they've decided to place the right for secession at that level, the state level, which seems to make the most sense for early USA. The right to secede could be argued as the right to set a non-republican or non-constitutional government, for example, and that could be the largest effect of this change.
Perhaps a native state could be admitted under different terms in this USA? Perhaps Hawaii is admitted with the local monarch acting as the governor?
So what happens if a pro-secession faction violently upends a state government or even a by the books "legal coup" does the federal government have the power to step in?
If they haven't laid out a process for secession they're asking for trouble when someone does want it.
The right of self-determination just isn't something governments can believe in. If they make it technically legal, they'd also make it impossible to accomplish (Catalonia is in a kafkaesque situation), or they'd consider it a hostile act from the new state (Brexit
sort of, and every proposed secession from the USA is assumed to immediately create a new and hostile neighbor).