Can you replace the Headless Quarters with a gallery build?

Yes! Or at least you should be able to. I replaced all of the active career locations from Get To Work. You have to build it on a separate lot, ideally of the same size, and then save to your library when complete. To place the new lot, you have to go into it through the active career, open up Build/Buy mode, and replace the existing lot from our library. I haven’t done this yet for the Headless Quarters, but this has worked before with no issues.