The change to a new civ will cause an OOS in every FfH mod (and FfH itself). Thus far unsolvable, but I am not sure anyone has devoted serious time to solving it. Once you have brought the new civ into the world, have the host save the game, exit to the lobby, and when the game reloads, select the new civ instead of your old one.
Playing past an OOS means that you and the other player ought to just be playing a single player game, because you no longer see the same information (ie - for you, the Luchuirp may be dead and out of the game, while for him they may be dominating the scoreboard and covering the lands. All because of your random numbers being shifted off from one another by a single roll 200 turns before and ignoring the OOS)