How to change Absence Carryover Expiration mid Absence Plan

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Hi,
When our Absence Plan started on 1st Jan 2021 the Plan had a carryover expiration of 3 Months - so Carryover expired 31st March 2021 and this works fine. A new requirement to extend this to 4 Months (30th April 2021) has been agreed so I have updated the Expiration Period to 4 Months on the Annual Leave Absence Plan but it is not reflected in Employees Absence Records.
I have rerurn the Calculate Accruals and Balances process with todays effective date and increment but this did not help. I also tried renrolling on the Plan - no change to the expiration date.
Is this possible and how to achieve?