If I have a paid subscription (with all my billing information) and If I change the UserID to another email, does the new email now have access to the account management tab? Can the new email change the billing info?
The Account Management page can be gotten to either through the Sheets UI or if you have the original purchase mail, there's a link to it there (see Where can I find my Account Management page?). So technically anyone with the link from the email can get to the page, but only the license-assigned user can get to it from within Sheets Add-on UI. So if you have the link currently from the email (even if you're not the user on the license), you can go change the Payer email (which is where invoices and notifications will be sent to in the future).
If you change the User ID on the license: