Auto fill from a drop down list

I am working on a mantinance work order form. I have a drop down list of part numbers. I have a field next to the drop down list lets call it (field A). What I would like is that if the drop down list is showing a part number the field A would display the word "Replaced". But if the drop down is blank field A would stay blank. Is there a way to do this with out writing a script with every part number in it for field A?