Opinions change.And there have been amendments to the Constitution before. (I seem to recall how much women had to fight for the right to vote. So I repeat: opinions change.)
Furthermore, that tweet of Stephens does not indicate whether he approved of the Second Amendment at the time: maybe he did, maybe he didn't --further checking needed! He was making a prediction.
And for those interested in a discussion, here's a good place to begin:http://www.nybooks.com/articles/1995/09/21/to-keep-and-bear-arms/
Yes, opinions change. Stephens's changed awfully conveniently.