Automatic conversion of a cell value and inserting specific value in another cell

hy,
when i paste workhours from monthly schedule, for ex. a1=12 (day shift), and b2=12 (4 hours of day and 8 hours of night shift) and so on for all days in one month (two rows), a need to run a macro to automaticaly change value of b2 cell to 8, and to enter a2 cell value 4... (to split night hours for calculating paycheck).. - only second row where there is a cell value, cell changes and cell above... two rows are for one person, than c,d for other, e,f for another and so on.

Thanks

i need to automaticly populate (or replace) night hours 12 into :
from 19.00-22.00 = 3 hours day
22.00-06.00 = 8 nightly hours
06.00-07.00 = 1 day hour

4 hours of daily shift
8 hours of nightly shift

i can copy sheet and change in it
have 300 people

................
i need to automaticly change 12 night hours to 4 in day and 8 in night in paycheck sheet from monthly sheet