There are very few agreements that cannot be changed if both parties are interested in changing it. However, if your agreement was made part of your final divorce decree, it would be up to the judge to allow a requested change after a lawyer officially presented the request by petition to the court. Even in the unlikely event that the court would refuse such a request, nothing would prevent the parties to the agreement from entering into another agreement which would recite that the new agreement takes the place of the old one.
If just one party to an agreement wants to change it and the other party does not agree to the change, the request for change would have to be submitted to the court by petition. A hearing would be scheduled and testimony and evidence submitted by both parties would be considered. Unless there is fraud, accident or mistake involved, the court is unlikely to permit changes to the agreement over the other party’s objection. The process of petitioning the court is expensive and no lawyer can guarantee success. In fact, having the court find a sound legal reason for allowing a change to an agreement when a party objects to a change is fairly rare. Generally speaking, claiming stupidity or finding out one no longer likes the agreement will not work and your lawyer would tell you that before taking your money to petition the court.
Of course, if one party has failed to act in accordance with the agreement or has acted contrary to its terms, the court stands ready to accept legal action to enforce the agreement. Still, doing so will typically require one to hire a lawyer. In agreements I prepare for my clients, I always include a provision that if one party breaches the terms of the agreement and the other therefore has to go the court to enforce it, the party breaking the agreement will have to pay both lawyers! That usually has the effect of both parties living up to the promises each made in the agreement. After all, who wants to pay for the other guy’s lawyer?