how can i change my live subscription from my account to my nephews account

1. No.

2. Sharing is done through the Home setting. If the subscription is on your nephew's account, he would want to make sure the console he is using is set as his Home console. Setting the console as his Home will remove it from he other console. https://support.xbox.com/en-US/help/hardware-network/console/my-home-xbox

3. If the subscription is on your nephew's account, he won't lose anything since it's tied to his account.

4. If your nephew deleted his account from the old console and the ex doesn't have any of his login information, then he shouldn't be able to access his account at all. If the account is still on the old console, your nephew would want to change his password.

I used this same method for my sons account without a hitch a few weeks back.

Once a birthday is set on the gamertag it cannot be changed. It will have to age normally and the parent account cannot be removed until he reaches 18.

Perhaps the reason you cannot change the email address associated with the account is because the birth dates do not match.

Hey there CAPP360,

One important thing to note is that certain actions invoke a 30 day security lock out on accounts. While in this lock out you will be unable to make certain changes to accounts including changing the Microsoft Account. You will still be able to use the account
normally, you just can't make further changes for 30 days. This is to help protect accounts, if someone should steal your information and start to make changes there is a one-month window for Microsoft to be able to stop them before everything gets changed
and there is no more information on the account to prove you own it.

One of the things that can create the 30 day lock is one you create a new Microsoft Account. You can use the new MA to make a new gamertag, you just can't move an existing gamertag to the new MA.

Right now it certainly sounds like your nephews account itself may be in the 30 security lock, but of course it could also just be the new account you created.

Hey there CAPP360,

Sorry to hear you're having issues changing your nephew's account.

For this particular issue, I would recommend getting in touch with our
Phone Support team. They'll be able to look into that account with you and work on sorting things out.