There's no way to put a field construction into the Replace With box of the Replace dialog, but there is a way to do what you want.

Somewhere in the document (or in another document in Word), insert the two fields {DOCPROPERTY WPFilePath}{DOCPROPERTY WPFileName} and then cut them to the clipboard.

Make sure that field codes are visible in your document (press Alt+F9 if needed). Open the Replace dialog and type the existing path & filename (without the quote marks) in the Find What box. In the Replace With box, enter the code ^c which represents the clipboard contents. Then click Replace All.

For future reference if you don't remember the ^c code: Open the Replace dialog, click in the Replace With box, click the More button, and then click the Special button at the bottom. In the popup menu, click the "Clipboard Contents" item to insert the corresponding code in the Replace With box.