(a) Except as provided in subdivisions (b) and (c), the provisions of an agreement for the support of either party are subject to subsequent modification or termination by court order.
(b) An agreement may not be modified or terminated as to an amount that accrued before the date of the filing of the notice of motion or order to show cause to modify or terminate.
(c) An agreement for spousal support may not be modified or revoked to the extent that a written agreement, or, if there is no written agreement, an oral agreement entered into in open court between the parties, specifically provides that the spousal support is not subject to modification or termination.