iCloud account

See this Apple article: Change your Apple ID -> Change your Apple ID - Apple Support (SG)

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If you are no longer using the email address that's associated with your Apple ID, you can change it. You won’t lose access to your contacts, purchases or other account information.

Then see this Apple article: What to do after you change your Apple ID or password

What to do after you change your Apple ID or password - Apple Support

but is there a way to do the opposite?

What do you mean with -> ❯ is there a way to do the opposite? ❮

Are you saying that you have a Yahoo account as your Apple ID, and you have already created an iCloud account?

If so, no, you can’t merge multiple Apple ID accounts.

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.

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)