If I buy game pass ultimate, will it cancel my game pass core.

You can cancel your current Game Pass and then just buy Core. Or you can buy Core in addition to Game Pass for console. Or you can buy Ultimate which includes both and will convert what you already have to Ultimate as well.

No. It wouldn't convert like that. You'd either just have to buy PC Game Pass separately, convert to Game Pass Ultimate in order to have both, or try to cancel and refund the Core subscription and then buy PC Game Pass.

If they redeem the code, then it would convert their subscription to Game Pass Ultimate, and cancel Game Pass Core.

You can only upgrade to Ultimate. If you were to add Game Pass for console, it would be considered a separate subscription. If you do want to just have that, you'd have to cancel your Core or wait for it to run out and then buy Game Pass for console.

Keep in mind, Game Pass for console does not include the ability to play online. If you do want to play online multiplayer you will need Core or Ultimate.

Also, if you purchase Game Pass first, then it would be converted to Ultimate and if you cancel ultimate, you would lose your Game Pass subscription. So if you do cancel Ultimate, do it first before you downgrade to just Game Pass.

Which subscription are you downgrading to, is it Xbox Game Pass, PC Game Pass, or Game Pass Core?

If Game Pass Core, you will only have access to just over 30 games that are part of the Game Pass catalogue, and online multiplayer.

If Xbox Game Pass, you will have access to all the Game Pass games on the Xbox Console, and no online multiplayer.

If PC Game Pass, then you will have access to all the Game Pass games on the PC including EA Play, and online multiplayer.