What happens at rpm -U .. if any file of the package being updated has been modified? Is it left updated with a new one, or the new (modified) file is kept with new extension?
RPM will overwrite any files that have changed unless they are explicitly designated as "configuration" files using the %config flag. Then it will make a copy of either the new one (*.rpmnew) or the old one (*.rpmsave) based on whether or not the config file is flagged as noreplace. See this reference for more details.