Swap email on X Box Accounts for a Child Account to set restrictions

Good evening,

I know there are several feeds in the community on this subject but I have never come across a definitive answer on this.

I, like many others set up an account on Xbox, using my email account for my eldest son to use, so that he could play Fortnite, use Xbox Gold, etc. etc etc.

I now would like him off my account and on to the account I have set up for him, already using his outlook account, so that I have parental controls, time limits, etc. but he has a huge amount of money and time spent on Fortnite etc., which I don't want
him to lose.

1. When I have tried to swap emails on the account before it has threatened to erase my email account from all history (or words to that effect). What does this mean?
2. What is the most pain free / least impactive means of doing this without him losing everything and for me to get my controls back?

Thank you gamers!

:)