Versions 4, 5 and 6 can all happily coexist on the same machine if you so choose.
We find that some customers get upset if the previous version is automatically uninstalled during an installation of a new major version.
Therefore, we let the customer decide if they want to remove previous versions. You can uninstall previous versions via the Windows Control Panel.
Whilst customers do not want all of the previous files automatically brought into the new version, you can easily add old files back to either via drag & drop of via the Import command in the main menu.