It is not supported at all. I tried doing this once, I think attempting to migrate 1.12 to 1.13, and could not get it to work. The tricky part is that you basically have to merge two databases that are not organized in the same way, since the newer version adds additional fields in some tables which must be populated as well as additional rows in tables which may or may not have changed schema.
I wouldn't recommend trying this unless you feel very comfortable with the DB organization already, otherwise it will probably be more frustrating than fun.