changing adult account into child's account with transferring achievements etc.

Unfortunately nothing can be transferred between accounts. In order to continue using the achievements, etc. on the original account, your child would have to continue using that account, but it would not be possible to use parental controls since it is
an adult account (adult accounts cannot be converted to child accounts). He would have to start over on the child account.

Hey there!

This is the exact opposite of the typical scenario we run into here! Normally all the child accounts wish to be adult accounts.

Unfortunately, there is no way to do what you are asking. The only way to have your son's account to be a child account would be to create a new one. Unfortunately achievements, game progress, games, etc cannot be transferred between accounts.

Let us know if you have any further questions.

Nothing can be transferred between accounts.

And can I change the adult account to a child account?

There is no way to make an adult account into a child account. You would need to create a new account as a child account for him to use and then your account would be the parent account that oversees it. There is no way to transfer any achievements or
game saves to that account.

Unfortunately you can't change an adult account into a child account and you can transfer items among different accounts. Just make a new child account for your child and associate your account as parental account.