Change my Apple ID

1.

Depending on what you did with your 'old' Apple ID.

If you have deleted it, it is no longer valid, it simply no longer exists and it no longer works.

To understand about a deleted Apple ID, see the Apple articles below:

How to delete your Apple ID account

How to delete your Apple ID account – Apple Support (UK)

-> Once your account has been deleted, Apple can't reopen or reactivate your account or restore your data.

Understand and control the personal information that you store with Apple

Understand and control the personal information that you store with Apple - Apple Support

2.

If you've ever requested to deactivate your Apple ID, see -> How to deactivate your Apple ID account

How to deactivate your Apple ID account – Apple Support (UK)

⤵️

Scroll to ->  How to temporarily deactivate your Apple ID

Sign in to your Apple ID account page on a Mac, PC, iPhone, or iPad. Then scroll down to Data and Privacy, and select "Manage your Data and Privacy." On the following page, select "Get started" under "Deactivate your account." If you don’t see this option, it means that this feature is not available in your country or region. 

👉 When you request to deactivate your account, you receive a unique alphanumeric access code. Make sure that you keep this access code in a safe place—you must have it to reactivate your account. 

If you forgot the access code, try to contact Apple Support -> Get Support (apple.com)

3.

If you still know that 'old' Apple ID, check your Apple ID via your email box.

See this Apple article: If you forgot your Apple ID

If you forgot your Apple ID email address or phone number - Apple Support

OR see -> Having trouble signing in?

https://iforgot.apple.com/password/verify/appleid

See -> If you can't connect or sign in to iCloud

If you can't connect or sign in to iCloud - Apple Support

B T W:

You are sign in here .  .  .  with your Apple ID & Password.

Your Apple ID is the personal account you use with all Apple products and services. It includes the email address and password you use to sign in, as well as the contact, payment, and security details that you use with Apple.

You can’t merge multiple Apple ID accounts.

What happens when you use the same Apple ID & Password that you use to sign in here?

See the following Apple articles

Sign in with your Apple ID -> Sign in with your Apple ID - Apple Support

Manage and use your Apple ID -> Manage and use your Apple ID – Apple Support (UK)