Just having a look back over older questions (only just noticed the question of the day on newsletters!).

I got this completely wrong but still think I was right at some point. I went with "Option 2 will fail" because I imagined that the loginname column would never be NULL. Therefore trying to switch it from NOT NULL to allowing nulls would cause an error based on the existing 50 million records surely?!? But then I remembered this was on older version of SQL (I have none to test on) and it allows it in 2008? Can anyone confirm that?