I've installed 6.3, and answer "yes" when asking to create backup I already had 6.21 backup from older version, so the installer of 6.3 had not backuped mirc.ini and others files because they already exists. What to do? - create a dir for backup of each versions? - overwrite old files ?
I've tested this several times here and the installer correctly backups and overwrites older files in the backup folder. Which version of Windows are you using? Are you absolutely sure that the installer is not backing up your current files?
I tested this by 1) installing mIRC 6.21 to a new emtpy folder, 2) running mIRC 6.21 and exiting, and 3) installing mIRC 6.3 into the same folder. I repeated several times, modifying various ini files each time to confirm that the backup was working.