Beginning from the CustomTools 2011 version, you are allowed to have an older version of CustomTools installed alongside with the newer CustomTools version.
When you start the CustomTools installer, the previous version(s) of CustomTools are automatically detected. If any older version(s) of CustomTools are detected on the installer startup, you get prompted by a message box to decide whether to install the new version onto the side of the earlier CustomTools version without removing the older version installation or to upgrade the existing CustomTools installation by the new version.
If you decide to upgrade your existing CustomTools installation, the old CustomTools version is removed from the workstation before the new version installation starts.