Yes, as long as the field has a value in the XML file (that is, you aren't talking about a new field that may not be present in the old XML). You will think I am crazy, but you can open the library in Explorer view (from the Ribbon) and then, copy all the files. Paste them in a local folder. You don't have to copy them back in, or anything - as the files paste into the local folder, the promoted props will update. Try one or two and you'll see what I mean. Then, when they have all copied over you can delete the local folder.
Hilary Stoupa
