Phew. Coulda got even further if I didn't make some rookie mistakes. xD
It's not so much a format change as I had a bug in my editor tools that kept leaving me with duplicate prefab IDs. This would disconnect a load of pieces when I tracked down and regenerated each item. I think -- think! -- I've finally cracked it, so future versions should finally be able to absorb earlier saves. Should. Hopefully. We shall see.