A loop can do this, but replace field contents can remove the need for any script at all let alone a loop if this is a one time fix:
1) Make a back up copy of the file just in case this produces results you didn't expect
2) Perform a find or whatever to pull up a found set of just the records that you want to change
3) Click or tab into the date field and select Replace Field Contents from the Records menu
4) select the calculation option and set up a calculation patterned after this example:
Date ( Month ( YourDateField ) ; Day ( YourDateField ) ; 2013 )
This will keep the month and day unchanged but change the year to 2013.
Thanks alot! Works like a charm!