Open alpha testing of the 5.0 HD mod (online lobby).

That's incorrect. Moreover, a game started without "Overlord Heroes" (OH) can be continued with OH by loading the save file.
The only thing that might not be immediately clear is that when playing with OH, you should always load the blue player's autosave. This is because the red player's saves (both autosaves and manual saves) do not save the blue player's turn (as that turn hasn't happened yet).
You can, of course, load the red player's save, but then the blue player will have to repeat their turn.