If mechs are just the sci-fi version of knights, then what would be the technology that renders the new knights obsolete? As guns improved and had increased reliability, penetration, range, and accuracy, knights gradually saw less and less use on the battlefield. So what would be the sci-fi equivalent of the 'gun' in this situation?

See Battletech and Warhammer 40k for examples of mechs being knights.

If you were forced against your will to make one major permanent change to the setting as a whole, what would it be?