I want to change the main email address I use my Apple ID

I'm not sure what you mean by "secondary" address. If you mean an alias, that cannot be used as an ID.

Add and manage email aliases for iCloud Mail on iCloud.com - Apple Support

"Note: You can’t delete or turn off a primary iCloud Mail address. You can’t use an alias to create a separate Apple ID, and an email alias can’t be converted to a primary iCloud Mail address."

Here's how you change the email address you use for your Apple ID:

1. Go to https://appleid.apple.com and sign in.

2. In the Sign-In and Security section, choose Apple ID.

3. Enter the email address that you want to use as your Apple ID.

4. Choose Change Apple ID.

5. If you changed your Apple ID to a third-party email address, check your email for a verification code, then enter the code. Learn what to do if you didn't get the email. --> If you didn't receive your verification or reset email - Apple Support

6. If you use services like iCloud or Messages to share with friends and colleagues, sign in to those services with your updated Apple ID. --> What to do after you change your Apple ID or password - Apple Support

For more details, read the document: "Change your Apple ID" --> Change your Apple ID - Apple Support

This doesn't work. It says:

You can only add the @icloud.com address that is already on file in your account.

Even though the new @icloud.com address is exactly the secondary email address already added to the account.

Yes, that’s what I meant. Thanks for clarifying it.