How about restoring the structure from the Saved/Autosaves/Game folder? Maybe your old working structure is there?
If that’s not possible, but you remember what changes you need to make to get the structure back to the old version, try creating a new empty project and copying your structure there. In the new project, change the structure to get it back to the old version. Save it. Then replace the structure in the broken project with this new one. I don’t know if this will work, but I think it’s worth a try.