I am not sure I understand your problem but I think you might want to try something like this

Set Century On
select table
dtOld = table.field
dtNew =YourProgram(dtOld)
Update table SET field = dtNew WHERE field = dtOld

or without going to an external program

select table
do while not eof('table'
if YEAR(field) <= nRollOverYear
dtold = table.field
dtNEW ={DAY(field)/MONTH(field)/YEAR(field)+ 100}
* You will have to add the time if this is a datetime field
endif
skip
enddo
Update table SET field = dtNew WHERE field = dtOld

I hope this helps