As the title suggests, will DA overwrite DL allowing only the expansion to be played?

Or as in the case of C&C Generals & Zero Hour, does the expansion install along side of the original allowing you to play either game as you wish, without the need to uninstall if you want to revert back to the original game.

I have found having both versions of the game (original and expansion) available to play without uninstalling anything to have a bit more life than just complete overwrites. Keeps things fresh being able to revert to the old game, as im sure some of us every now and then still put up GC1 for a while too.