I had an installation of the drm-free version of Worms Reloaded, and I didn’t want to re-download the Steam version that has multiplayer, so I copied the installation of the humble bundle version to the Worms Reloaded folder in Steamapps. Then I uninstalled it in Steam and reinstalled it, Steam said “Discovering existing files” and it began to download the missing files. It said it was 700MB but it only downloaded the ~3MB it needed.
tl;dr copy existing game files to steam, and steam downloads the difference