Perform a find for the year to find all records of a given year or range of years. (You can use < 1950, for example). Use Replace Field contents with this calculation to correct the year values:
Date ( Month ( theDateField ) ; Day ( theDateFIeld ) ; 2000 + Right ( Year ( TheDateField ) ; 2 ) )
use the name of your date field in place of "theDateField"
Oh yes, try this on a copy of your database just in case you make a mistake. It can change a lot of records and can't be undone.