When you're using optimistic row or table buffering the is no difference between the update and replace command because the values are only written to the table when you close or update the table. No record locks are in affect when you replace or update some records. This is delayed untill you commit your changes (by closing it or a tableupdate command)

NOTE when your using replace to replace some values in an other workarea than the current, be sure the table in the current area is not at the end of the file (EOF()) because the replace would not have an effect