Actually, you can rename your promoted property columns without changing your data source names, I think. Under Tools / Form Options / Property Promotion, you can modify a promoted field, and type a new name for it in the column name. I'm not sure if your existing xml will have to be reopened and saved to then display their data in the newly named columns -- you may want to test this first by publishing a test form to a library, creating a few xml files from it, modifying your column names in the template and republishing.