How can I recover a old iCloud account without messing up my current one

If the old account was set up with 2FA and you only remember the Apple ID and password is forgotten recover the account see the article How to use account recovery when you can’t reset your Apple ID password - Apple Support

If the account was set up with secondary authentication , and you have forgotten all account credentials like three answers for security questions , date of birth , rescue mail address and its password and the Apple ID is not signed in the device contact apple support with credit card details in case added Contact Apple for support and service - Apple Support