Migrate Xbox Account (Gamertag) from One MS Account to Another

I have two Microsoft accounts, A and B. I created A when I was a little kid, so it has a corny, childish name (by this I mean the name preceding @hotmail.com); and, evidently, I used this account for my first Xbox ever, and all of the Xbox's that followed. This A account has many bought games and many progress on different games, including achievements that I cherish deeply, but maybe the most important thing is the bought games. As I grew older, I realized that using my corny A email was not an option for professional environments, so I created account B and stopped using account A for everything but for Xbox. Years went by and, recently, my A account was blocked "due to security issues". I filled the security questionnaire about ten times in a span of one month, being certain that I was providing correct and precise information. However, just a few days ago, Microsoft decided that the information wasn't enough to let me access the account, and that it will be permanently blocked; the email with this notification even stated that the best course of action was to create a new account!

I've seen some other questions and posts regarding similar issues where it is stated that there is no way to link one Xbox account to a MS account different from the original one, but I still want to vent out my nonconformity with Microsoft's security procedures; I understand this is to prevent strangers to get into our accounts, but this means that all the time, money and effort put into my account A is permanently lost without a way to appeal or fight for the things I've bought with that account.

So, any idea on how to recover my gamertag from account A to B?

:)