Microsoft was almost broken up because they bundled Internet Explorer with Windows during the "browser wars". Since then Microsoft has learned what to do and what not to do when dealing with the government. I don't believe for a second Microsoft is going to sacrifice all the goodwill they have built up over the years just to pull a "gotcha" and make CoD exclusive.
Yeah I doubt they would but I just wonder where a name change would sit legally. I know the name is the IP and a change in name would erode somewhat the very thing that gives it value but, companies rebrand all the time without it causing too much of an issue. We have also got the FIFA situation with EA where if they don't renew the contract to use the name then they are going to have to go through a similar exercise of renaming.
If MS were planning to do something like this then they could do it over a fairly long period of time so as not to damage that good will they have built up, something like;
Call Of Duty Ultimate Soldier (multiplat)
COD Ultimate Soldier 2 (multiplat)
Ultimate Soldier 3 (exclusive)
Personally though I think that the fact MS have already said that they aren't going to fully integrate ActiBlizz into Xbox Game Studios but going to run it as a separate arm alongside speaks volumes. It means that at some point they could resell the whole thing on and keeping it seperate to the rest of the company lessens the risk of the initial purchase to some degree. They have made a huge investment buying ActiBlizz and they will want the monitory value of it to remain as high as possible should they choose to sell it. Losing playerbase due to making games exclusive does not achieve that. It's one thing to spend millions on a AAA developer and make them exclusive, it's another entirely to spend billions buying one of the biggest publishers with multiple development houses and some of the highest grossing games in the industry. Just the simple fact that Microsoft will be in control and be able prevent anyone from making marketing and exclusivity deals with ActiBlizz is massive and probably enough to justify the purchase.