Although the upgrade from a previous version of eVo to v21 is semi-automated, no equivalent roll-back is available. Should you wish to install a an earlier version to a server with v21 installed already, v21 will need to be fully uninstalled and a completely new installation of the chosen earlier version made.
No configuration, setup, form types or tasks will be transferable as it is a fresh install, other than by use of the standard backup mechanism.