Technically, the ACC owns the GoR agreement with the schools and ESPN pays the ACC for the right to broadcast their games. If FSU moved to the SEC, ESPN can't air them, even though they hold a broadcast agreement with both, unless the ACC approves that use of their rights.
Some negotiation or "make right" would need to occur, because the ACC will lose value with FSU gone, as will ESPN, since their contracts and the agreement to build the ACC network was with FSU involved.
If FSU left next year, they'd owe a buyout (which in the ACC is 3 years of revenue) then ESPN would trigger a composition clause, informing the ACC that their revenue will decrease by $X. Then the ACC would look to FSU and say, "There's your number. We'll hand you back your rights once we're all even"
It took the B12 a year or so to figure out a solution and that was in the last year of the GoR, i still struggle to see how this will be an easy move for FSU.