Is cave adaption really permanent?
It seems to me that it isn't. My military spent almost all their time indoors before being drafted, but after a few years of being stationed outdoors ~3/4 of the time they don't seem to be stunned or vomiting.
Maybe I just haven't caught them in the act.
I don't have the screenshots to prove it, but perhaps you can take my word. I once had a fortress where pretty much everyone was cave adapted. I built a meeting hall with wells and exposed the ceiling to sunlight (I also built a glass ceiling to keep unwanted stuff out). For a long time, the floors were painted green. After a good year or two, though, the vomit was not replaced, and dwarves were not seen being stunned in the meeting hall. It wasn't sudden--over time more and more dwarves became irritated instead of nauseated until they were all comfortable.