Changes in the rate applicable to an open end account, including a credit card plan which provides for sales, cash advances, or both, are limited as follows:

(1) Any balance existing before July 1, 1982 is to be repaid at the then applicable rate of interest or finance charge regardless of any subsequent increase in the rate applicable to the account.

(2) If the rate applicable to any balance for which the borrower or buyer becomes obligated on or after July 1, 1982 is increased, the borrower or buyer may repay the existing balance at the rate in effect prior to the time the increase becomes effective. If the borrower or buyer increases the balance of his account by making purchases or requesting cash advances, the increased rate shall apply only to the portion of the balance incurred after the rate increases.