I recently activated my 2007 physical copy of Supreme Commander on Steam that has been installed via disk since that time. It gave me access to Supreme Commander AND Forged Alliance (yay free game! :D) but according to Steam I have to re-download and re-install SupCom, even though it is already installed.
Is there a way to move the original game files around such that Steam knows it’s installed? Yes, I know I can uninstall the already installed copy and re-download it but I’d like to avoid that if I can.
To give a simple answer, you have to redownload it. It’s possible for some games that the files could be the same, and in that case, you may be able to start it downloading, then exit Steam during the download, move your files in, then reopen Steam and resume your download. It’s possible it may notice new files. However, it’s most likely that won’t work, and to keep things simple and your game files non-corrupted, I highly recommend you avoid the hassle and just redownload the game.