IMessage is asking me to sign in with an old email address and is not letting me change it

Hi PJtoday,

It sounds like you're having an issue with using your current Apple ID for iMessage on your iPhone. If so, is iCloud for Messages on? If yes, are you signed into the correct account for iCloud? You can check with the steps below from the article here: Manage Apple ID settings on iPhone - Apple Support

"Sign in with your Apple ID

If you didn’t sign in during setup, do the following:

1. Go to Settings .
2. Tap Sign in to your iPhone.
3. Enter your Apple ID and password.
4. If you don’t have an Apple ID, you can create one.
5. If you protect your account with two-factor authentication, enter the six-digit verification code."

If you're seeing the correct account check to see if you can deselect your incorrect Apple ID account with the steps in the article here: Set up Messages on iPhone

Should you continue to have an issue, check and complete the steps in the section "If you can't see or select your number in Messages or FaceTime" here: Add or remove your phone number in Messages or FaceTime

If you're still unable to change or sign into the correct Apple ID to use with Messages, reach out to Apple Support directly to further assist. Get connected using the link here: Get Support

Cheers.