@funkeeferret, what I did was load the original and strip it down to the walls and build it up again, checking that I was only using base game pieces. I have a utility that turns off expansions and stuff packs, but it can't disable store content. There is "usually" a small icon in the thumbnail to indicate store content, but apparently either I overlooked it or it was missing from a couple build items. They showed up in the filter when I uploaded the lot to my Store Studio.
It actually worked out for the best because once I got back in I had another look around and made a few more enhancements. So, the finished build is a bit better than it would have been. Now my problem is that my computer crashed as I was working on it. Not a big deal for the build; I remembered precisely what changes I had made. But, I am now unable to sign in to my accountant, either in-game or in the Launcher. So I can't upload the lot to my Studio. This has me a bit concerned because it may be a sign of a larger problem. Heck, my computer crashing at all is concerning. It's less than a year old and I'm very careful about what I put on to it. Even if The Sims crashed, it shouldn't have brought the whole system down. But that's a different issue.
So, for now I'll post the lot to Simshare and move it to my Store Studio when I can. So.....