In its current implementation, MythTV is only compatible with the same major version. This comes into play in when you have multiple systems in your network that all run MythTV (ie. MythTV 0.22 systems are only compatible with other MythTV 0.22 system, not 0.21 or 0.23 systems). In other words, if you have a MythTV backend running 0.21, and decide to install a new computer with MythTV 0.22, you will need to upgrade your backend to MythTV 0.22
Upgrading for more info.