Customarily, we would append a suffix to the existing username like "_old", or "_old2". The latter, or something equivalent, would be used in case the name with the _old suffix already exists, as many namechanges have been done over the years. For example, peanut_old exists already, so if someone wanted the name "peanut", we'd opt to change the current account with that name to "peanut_old2" (but for the record, "peanut" is currently unavailable.)