How can I change my Apple ID iCloud alias/email address?

Hello everyone,

I used a gmail address to create my Apple ID years ago. Apple gave me an iCloud alias identical to my gmail alias back then!

Now I've been using a new gmail address and have changed my "Apple ID" email (gmail) address accordingly. But I still see that old iCloud alias on my Apple ID everywhere in Settings, Mail, etc.

I even created a new iCloud alias identical to my new gmail address but it doesn't consider it as my "Apple ID alias". This is very confusing for me and I hope experts have a solution for this.

Thanks in advance