By the way, it seems like the Steam version is still receiving updates like this one:
http://forums.totalwar.com/showthrea...tch-05-09-2014
Quote: "Wrong sized siege equipment will no longer be deployed in siege battles."
Now Steam also has a bad reputation for making it hard to impossible to sell game activated through it and not giving you an option to return to an old version of a game if an update breaks things for you. You can however pause updates until you are done with a playthrough if you are worried about an update invalidating your savegames. I'm pretty sure Creative Assembly are aware of the savegame issue, but don't say you haven't been warned.
Now if you want to switch to Steam, go to your library and click on the "+ add game..." button. There you will be asked to enter your CD key and the game will be available afterwards for download in the library. The same can be done for the Kingdoms expansion which is now called a DLC (down-loadable content). Since Steam uses predefined installation directories you can uninstall your retail version of Medieval II + Kingdoms before or after installing the Steam version. Since savegames are local to the installation directory (unlike other games that may use a per-user directory), the Steam version wont pick them up unless you copy them over. And whether the Steam version is savegame compatible at all, I don't know.
But hey, at least Medieval II has a kind of long term multiplayer support now that it is on Steam!