Did anyone else receive a strange message from Steam last month about their file format or something changing and that Steam needed to reinstall Skyrim?

If so, does it remove everything including mods? Does it take up any extra hard drive space? I have very little HD space right now, plan to fix that with a new hard drive.

I'd rather not reinstall it, especially since I don't get why Steam wants me to reinstall it. I can run Skyrim perfectly fine through SKSE.

But, what exactly is Steam going to do? Replace the whole game or just certain files? I'd appreciate an answer, since Steam support seems to be "Herp derp deeeerrr" at the moment.

Please post the exact message verbatim, with a screenshot.