At any time during the duration of the contract but before the beneficiary graduates from high school, the purchaser of a prepaid tuition contract for a senior college plan for four years or a private college plan may purchase a supplemental contract to prepay the tuition and required fees of the beneficiary for 32 credit hours for one additional year of education, in addition to the undergraduate credit hours included in the primary contract.
34 Tex. Admin. Code § 7.53