I'm not sure if this will work with your account scenario, but I have done a similar in the past with some accounts of my own.
If you already have an old and new microsoft account, migrate the xbox gamer tag to the new account.
- Log in with the account you want to change
- Go to Settings and select Account
- Scroll right to Your Information and select Account Security.
- Select Microsoft account
- If you already have another Microsoft account, select Yes, otherwise create a Microsoft Account
- Enter the password for the Microsoft account that is currently
associated with your gamertag and select Sign in
- Enter the email address and the password for the new Microsoft account that you want
to associate with your gamertag, and then select Sign in
- Select Yes,
change to change the Microsoft account associated with your Xbox
- And Finally, Select Update Contact Information to
update your contact information or Done to finish
These are the steps that I took to transfer phone contacts to another Microsoft account. I did not have two existing accounts though and do not know how it will work in your exact scenario.
One caveat is that you will have to re-download all content manually. When you go to the store, the buttons will still say buy, but once you tap them it will provide a message to the effect of "You have previously purchased this already, do you wish to download?". You will not be charged again.