So everyone that has disc have to buy the game again?

What are you talking about? The more info you give, the better info we can give you.

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Hello everyone

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If I already installed a game and it is up to date (but it is a disc copy) and the digital version becomes available for super ultra cheap.

Will the game re-install itself again when I buy the digital version?

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Have a nice day or afternoon or night everyone

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Hello everyone

If I already have a disc game installed to the HDD and the digital version of that game goes on sale for super cheap, can I buy the digital version so that I can avoid having the need to put the disc in when I want to play it?

Have a nice day or afternoon or night everyone

The only way to get your disc game working on the "All digital" edition of the Xbox One is to purchase the game digitally.

It's not possible to "convert" a disc game to a digital game. Otherwise you could buy a disc, convert it to digital, and sell the disc to someone else who would then convert to digital, sell the disc and so on, until the whole world has the game digitally,
and only one disc was sold.

So the only way to get a digital copy of your disc based game would be to buy the game digitally again.

Not everyone has a fast internet or unlimited connection to digitally download games. Most people I know still goes and buys disc copies of games.