Games saved in older version of SC2 cannot be loaded without downloading the appropriate version and restarting SC2. For campaign missions, this is understandable in cases when campaign data, core, or something directly related to campaigns was changed. However, the last update brought only new campaigns-unrelated achievement, portrait, and a new map pool for MULTIplayer, so nothing related to single-player campaigns. If I remember correctly, long time agou this was not an issue and campaings-unrelated updates did not force to restart SC2 in an older version when an older saved game was loaded.
For stronger punnishing for saving games, the game is so stupid that after finishing a mission in forced older SC2 version, it is unable to restart to the last version smoothly and logs the user out.
It would be very very pleasant, if the campaings-unrelated updates would not prevent to load older saved games in the last SC2 version, or at least, if the forced CS2 reloading between versions was smooth in both ways.