The base game will be replaced with the deluxe edition.
Most often you wont be able to replace the game though :(
Some games have a way to upgrade the version to deluxe :)

If you would love to buy the game real badly you could try this >>

Permanently remove it from your Steam Library.
Make sure you have Steam Cloud turned on for it beforehand.
Then just add Deluxe Edition and your save-file(s) will still be there because Steam Cloud remembers.

You can also just copy-paste your save-file(s) to another folder or your desktop and just insert them back again afterwards.

No need to make it complicated.
If you buy or activate a key for a game edition that is different from yours, lets say you own the base game and buy the version that includes all the DLC it will just give you all the missing content/DLC and no second copy of the base game.

Thats how it always worked on Steam.
Also the tip above about removing the game from your account does nothing, if you then try to rebuy the game it will tell you you already own the base game and if you want to restore the license.
Steam stopped letting you rebuy "removed" games a few years ago because its pointless.
Since its really just an advanced hiding feature and the game license is still linked to your account, just temporarily deactivated.

引用自 Ogami：

Thats how it always worked on Steam.

It was not always like this 🤔

引用自 V3LTA：

It was not always like this 🤔

Its been that way since i joined in 2011. So lets say, it worked that way for at least 10+ years.

引用自 VividDreams17：

I want to buy a legendary version of dragon ball Z kakarot and I already have the base game. So I just want to know what would happen to the base game or if I would even be able to do that.

The Legendary Edition is just the base game plus a bunch of DLC.

You would get a license to the Legendary Edition and that in turn would grant you everything that it includes which is the base game and the list of DLC that belong to it. Since you already have the base game, you would obtain all of the DLC that you are currently missing.

You would be charged the full amount even though you already own the base game. The publisher would need to sell an upgrade or a Complete-The-Set type bundle that would allow current owners to save money on the parts of the game they already have. You would not get a 2nd copy of the base game.

引用自 Ogami：

No need to make it complicated.
If you buy or activate a key for a game edition that is different from yours, lets say you own the base game and buy the version that includes all the DLC it will just give you all the missing content/DLC and no second copy of the base game.

Thats how it always worked on Steam.
Also the tip above about removing the game from your account does nothing, if you then try to rebuy the game it will tell you you already own the base game and if you want to restore the license.
Steam stopped letting you rebuy "removed" games a few years ago because its pointless.
Since its really just an advanced hiding feature and the game license is still linked to your account, just temporarily deactivated.

Thank you for clarifying. Looks like I gotta do it mysellf now, Enough of researching ig.

This thread was quite old before the recent post, so we're locking it to prevent confusion.