Having to pay full price for a game i already have again

That's perfect then. Kindly remove and re-add your account and then try launching the game, check if there will be a prompt or notification for an upgrade.

REMOVE ACCOUNT​​​

​​​

1. Press the Xbox button to open the guide. ​​​

2. Select System > Settings > Account > Remove accounts. ​​​

3. Select the account that you want to remove, and then select Remove to confirm. ​​​

4. When you're finished, select Close. ​​​

​​​

POWERCYCLE CONSOLE​​​

​​​

1. Turn off your console by holding the Xbox button on the front of the console for approximately 10 seconds. The console will turn off. ​​​

2. Turn your console back on by pressing the Xbox button on the console or the Xbox button on your controller. ​​​

​​​

ADD ACCOUNT​​​

​​​

1. Press the Xbox button to open the guide. ​​​

2. Select your gamerpic in the upper-left corner of the screen, move all the way down and select Add new. ​​​

3. Enter the Email address and the password.​​​

4. Read the Microsoft Service Agreement and Privacy Statement, and then select I Accept. ​​​

5. Follow the steps to configure your Sign-In & Security preferences to add your Microsoft account to your Xbox console.​​​

You're welcome and thank you for your kind understanding.

I've seen some people being able to upgrade the game's edition through the Microsoft store and it was 70% off for them. For me it's still full price

That can be related to your issue. Make sure that you're signed in under the account where the game was purchased.

Kindly sign in to https://account.microsoft.com/billing/orders and then make sure that the Call of Duty Modern Warfare II Cross-Gen edition is showing up as "completed"/

It doesn't show up there. It only shows the cross-gen edition which i already have. Is this a Microsoft or Activision issue because im lost since the Microsoft store says i dont own it. Also a few days ago it said that i owned it and the vault edition was 70% off but idk why this happened