Annual accrual is not changing after changing of employee grade

Summary:

Annual accrual is processing based on employee grade, for grade 1 employees the accrual is 30 for grade 2 employees the accrual is 24. There is one employee whose grade is grade 2 and he getting 24 leaves yearly, but his grade was changed in month of march to grade 2, so from march the annual accrual should be 30 but while running march month accrual the balance is showing 24 only instead of 30. And in this year the employee didn't taken any annual leave yet so that means his balance should show as 30 from march month instead of 24.

Let me know how to resolve this!!

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