Leave Quota Reduction - Prorata basis (From Backdate)

Hi,

Leave Quota Per year = 17

Leave quota generated per month = 1.41 (fixed)

This quota has been generated from Jan-10 to May-10.

Now policy has been changed w.e.f. 01.01.2010 (Back dated)

New Leave quota per year = 12

Leave quota generated per month = 1.00 (fixed)

How to reduce leaves already assigned to employees.

Right solution will be rewarded full.