Well, tbh, they just use an email as a username a lot of times. As long as you note the old email address and password for an acct you should be good. Another option: Forward those emails with keys and other important info to a new address, or better yet, archive those all to a .PST (if you have Outlook for example) so they're available offline.

Last but not least, stop using that old email, switch all important stuff over and leave the old dormant. I DO recommend a strong password update and possible MFA since you'll seldom access it. But then you can disregard the old account and it exist for historical purposes.