[Suicide Junkie]

You've got it.

1) Players play their turns using the old version of the mod.
2) Then you host using the new version.
3) Then the players play using the new version.

The one thing you must not do is reorder items, insert or delete them. You can edit parts of an item, but wholesale item move/add/delete will break savegames.

Items can be added to the end of the file without breaking the savegame, but it may look sub-optimal in game, and they may not hide obsolete items properly.