You can simply change the original email address on the first account to an iCloud email address set up for that account***. You won't be able to use the one you just used to set up the new account*, plus you will have to wait 30 days before you can finalize the change.**
Here's how you change the email address (Apple ID) you use for your Apple Account while retaining everything in your account:
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 old ID 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
**- There's special instructions if you want to change your Apple ID to an @icloud.com address.
"If you try to change your Apple ID to an @icloud.com email address that was created within the last 30 days, you will be asked to try again later."
*From: How to delete your Apple ID account - Apple Support
"Can I use the primary email address for my deleted Apple ID account to create a new Apple ID or with another existing Apple ID account?
No, once your Apple ID is deleted, you can't use the email address that you used with it to create a new Apple ID. In addition, you can't use that email address with another existing Apple ID except as a rescue [notification] email."
***
Create a primary email address for iCloud Mail - Apple Support