At the moment, I don't believe that we have an event-driven way to know when changes are made to already placed furniture by either the player or other addons. This has proven a little problematic for my Oops I Did It Again add-on that provides multi-level Undo/Redo functionality.
I think (?) the only way that I can even detect when a player makes changes to already placed furniture is by tracking the item that was being placed at the start of a Mode Change and subsequently comparing the initial state (position and orientation) of that item to its state after another Mode Change event.
If it would be possible to have an Event that is fired whenever any change to an already placed item (including the FurnitureId, similar to the EVENT_HOUSING_FURNITURE_PLACED event) is requested via the Stock UI or the API, it would simplify this entire process and could greatly improve the compatibility between housing addons. I'm not sure if this was the intention for EVENT_HOUSING_EDITOR_REQUEST_RESULT; but it seems as though this event only fires for placement and removal of items -- not the changing of position / orientation of already placed items.
(and, of course, it would also be a total bonus if there were a separate event for whenever a change is successfully made to already placed furniture by another player -- seeing as the change is visible in the house, just not captured by the addon eventing -- that would even more awesome )