An association, in exercising its rights pursuant to a variable interest rate clause to increase the interest rate, may at the option of the borrower (a) increase the amount of monthly payment of an amortized loan, or (b) extend the maturity date, or (c) a combination of both (a) and (b), provided the loan will be paid in full in regular monthly installments by the end of the maximum amortization period allowed at the time the instrument was executed or for such additional period as may be required to amortize the loan without increasing the existing monthly payment, but not to exceed a maximum term of 40 years. An association may provide in a note issued under either Civil Code Section 1916.5 or Section 1916.6 that, upon a decrease in the rate pursuant to a variable interest rate clause, the association may at its option (a) decrease the amount of monthly payment of an amortized loan, or (b) reduce the maturity date to the degree the maturity was previously extended, or (c) a combination of both (a) and (b).