Well, I've had Metro2033 uninstalled for a very long time now, surprisingly enough. I decided to maybe try and replay it again, to prepare for the horrors of Last Light which is closing in. And then, at the beginning of the download, this came up: New content? What?
I had the same last time I played it, but mine was still in my PC. don't think is anything to worry about. mine works fine after the 'conversion' or whatever that is.
It's nothing unusual - games on Steam do that lately. It's worth the download since it only replaces what is there already and doesn't add anything (so you don't lose any more hard drive space).
Yeah - this is happening to all steam games I have played. They must have converted something on their end.