The Supreme Court has held that, when a marital settlement agreement provides for reviewable maintenance, the party seeking a modification does not bear the burden of proving a substantial change in circumstances to justify modification of maintenance. Blum v. Koster, 235 Ill.2d 21, 35-36 (2009). Instead, the trial court "ha[s] discretion to continue maintenance without modification, to modify or terminate maintenance, or to change the maintenance payment terms." Id. at 36. In exercising its discretion, the court should consider the factors in sections 504(a) and 510(a-5). Id.