You have changed Apple ID address , it has to be updated What to do after you change your Apple ID or password - Apple Support
Open safari preferences > password section > enter admin password , select the old Apple ID that shows as a suggestion and click on subtract sign , the password and website will be removed from icloud keychain ( you have to click on delete password ) .
Now add your existing Apple ID that is signed in Mac system preferences , click on plus sign , add the website , in the user name type the existing Apple ID address , in the next field add its password and click on save password .
Again open safari preferences > password section > enter admin password , select the website and autofill the password, the password and complete website address / Apple ID are now saved in iCloud Keychain .
So , whenever you sign in at www.icloud.com with fresh credentials the account will be accessed .
See the article Autofill your username and password in Safari on Mac - Apple Support