The new state should follow the guidelines of what the decree says but she can choose to hire a lawyer and see if she can get it extended. Usually it states that child support ends at the age of 18 or graduation of high school whichever is later. So be careful with that and make sure of your wording. Notify the state when the child reaches this age and provide a copy of the decree. Unless she takes you back to court to have it changed, that should stand.

The old decree is standing, but not on much, because it can be upset immediately by anyone who wants to enforce the law of the state jurisdiction where the child now resides.

That is the law most interested in the child and the welfare of that child. If the new state law says up until college, then that's going to be the law for the support.

The only way you could stop the support at 18 would be if all parties agreed to waive their right to continued support. Even then, this could be challenged later on, on the ground that the child wasn't fully mature yet and therefore not completely informed of the consequences that might happen if support were to end before college, thereby depriving the child of a chance for advanced education.

The existing decree governs unless specifically modified. However very few states require support beyond age 18.