^^^^^
According to MikeO, they can't leave the city. Their lease is ironclad.
Forgot the exact year but its a while before they can move, like 2029 or something. There is a loophole though, where the Jags can play HALF their home-games in a different city if they want (ie London, Berlin...etc) as long as half the homegames are played in Jax and the team name stays "Jacksonville"
You will see that happen before they move.
Jax went in "default" on their lease last year hoping the city would terminate it after Jax paid the fine, the city didn't terminate it though and didn't ask for Jax to pay the fine they let them slide...cause they want to keep the team. The team is stuck there unless a miracle happens or a lot of money is passed around to make the city very happy.