Lets say one has a legacy username and the email linked to the acc is one i made an alt with. What would i have to do to use the new Jagex account system?
When you upgrade your characters/accounts to the new Jagex Accounts, you can choose any email you want to be the new login email.
It doesn’t matter what the previous logins were or what the previous linked emails were, you are setting up a new account.
The email you choose to be the new login email will also become the new linked (registered) email.
The Support Centre has an information page and a FAQ page about Jagex Accounts.