In a word, no. It would take nearly as much effort to retrofit Gettysburg as it would to create a whole new game. Plus we would have to rewrite the AI to have the cavalry act correctly for the ACW era.
-Jim
"My God, if we've not got a cool brain and a big one too, to manage this affair, the nation is ruined forever." Unknown private, 14th Vermont, 2 July 1863