HoMM3 HD is a mod (add-on) for the game Heroes of Might and Magic 3.

We are playing a game on 3.45. When joining the game, this message pops up:
http://gyazo.com/dbb5e53ac7ae8b33bc84407d8f2c8cf4

About two weeks ago we played and everything was fine.

Well, that means the opponent either doesn’t have HD+ or they updated their version. Too simple. The opponent has HD+ and the same version. Well, if both have 3.45, and suddenly the mod sends one of the players a
message that 3.48 is needed, then that’s magic.

Maybe it’s worth implementing a tournament patch that incorporates all the HeroesWorld rules? (for example, it would be physically impossible to upgrade fairies or make more than 2DD, etc.) How technically feasible is this?

Very feasible, as a plugin module for HD.

I don’t know what m200 is, but it too can be made as a plugin module for HD (if it isn’t already).

Right now, I view HD+ in terms of gameplay changes as a transitional variant from SoD to HotA. Because HotA includes all the changes from HD+.