The transaction goes ahead as planned.
Microsoft then have to sell off the element that they've agreed to with the CMA. It's yet to be determined what that would look like.
Hypothetically, they could come to an agreement to sell off the CoD IP, Infinity Ward, Treyarch, Sledgehammer and Raven.
This collective would then be sold by Microsoft to another company. Microsoft are not allowed to own this collective.
Could the new collective voluntarily choose to only make Xbox games? I don't know. It wouldn't make much sense for the future acquirer to ignore Sony so it's something that's extremely unlikely in my opinion.
Other people here know more than me so please feel free to chime in if I've got anything wrong.
I don't think the transaction goes ahead "as planned" as such - the divestment deal has to be done and transacted simultaneously.
I'm not sure if there's a specific legal part to that from the CMA, but from the perspectives of the participants it has to be this way.
CMA can't approve a deal without the divestment issue resolved. Buyer identified, divestment scope confirmed, contracts complete.
MS can't resolve it unless they have a buyer, a price and a plan for the leftovers after the divestment scope is dealt with.
MS also needs to be able to negotiate a fair value price - no one would offer fair value if MS are compelled to sell and MS would in the meantime carry the cost of an organisation they cannot legally interact with. So MS need to be able to walk away from the deal and they can only do that prior to completion.
The buyer can't agree to buy while MS doesn't have control of ABK. Conversely, the CMA can't wait for MS to take control and then monitor the sale process - too much shenanigans possible.
ABK also needs to know that following the disembowelment by the divestment, the deal will complete so they won't divest before the purchase day in case it all fell through.
All of these competing interests and interlinked contracts mean the purchase and divestment must all go through simultaneously - if any part aborts, it's all halted.
I'd expect the deals to all be signed up and a time/date of execution to be chosen when all the ownership changes are triggered simultaneously. And all of that would be approved by the CMA first.
This is probably why MS is quiet right now publicly at least - working out suitors for the divestment, working out price and other bits. Then they can respond with a plan or pull out if they're unable to find a suitable home for the divested stuff.