(3) Where the Court decides to alter, by order under this section, an agreement by inserting provision for the making or securing by one of the parties to the agreement of periodical payments for the maintenance of the other party or by increasing the rate of the periodical payments that the agreement provides are to be made by one of the parties for the maintenance of the other, the term for which the payments, or, as the case may be, so much of the payments as is attributable to the increase, are or is to be made under the agreement as altered by the order shall be such term as the Court may specify, but so that term shall not exceed—
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(a) where the payments will not be secured, the joint lives of the parties to the agreement or a term ending with the remarriage of the party to whom the payments are to be made whichever is the shorter;
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(b) where the payments will be secured, the life of the party to whom the payments are to be made, or a term ending with the marriage of that party, whichever is the shorter.