How carryover will works if we change the plan term in middle.

Summary:

We have one absence plan called 'Annual Leave' for this plan term is calendar year(01/01) here the balance will be carryover to the next year Jan 1st every year now the concern is we have few changes in the carryover and those should be effective from October 1st onwards and again it should work on continuous years like Jan 1st year for this time only one time it should work on October 1st so I changed the plan term as (10/01) but the balance is showing incorrectly because of plan term change in middle and it adding all the previous