I struggle to see what standing the new board has to sue and on what grounds they would try to sue on. There's essentially a contract in place and contract law puts the burden of proof on someone trying to avoid a contract. Essentially the only way they could do it is if the contract was not legal at the time it was signed, or if there was a breach of contract.
In this case, the agreement looks to be legal according to Florida law. As was said earlier, these types of agreements are common, and are specifically called out in Florida law.
As far as breach of contract, they would have to show that Disney did not live up to any provisions in the contract that place constraints around Disney.
Other than PR, making a lot of noise, and wasting a lot of money ( which is probably the point ), I don't see this having any teeth.