Changing the Apple ID on my older MacBook Pro

It sounds like you are trying to use an Apple ID that is, or was, in some way, associated with another Apple ID of yours (e.g., as a reachable at email address or a notifications email address). Note that you can't use such an Apple ID for an uncertain period has passed --sometimes reputed to 30 day and sometimes 1 year. Also, I have seen that you never can use such an email address again (e.g., if the account was deleted). The last two times I saw someone claim a time period it was “up to 30 days.” I believe that up to 30 days is the current answer. But, you can contact Apple support to check out the currently quoted wait period.

Apple ID Support https://support.apple.com/apple-id

Apple Support Twitter  https://twitter.com/AppleSupport There may be a wait.

Apple ID account security for country-specific andregion-specific help https://support.apple.com/en-us/HT204169

Get Support: We’re here to help https://getsupport.apple.com/. Go through the options to find the best support option.

You want to change to a current email address? Note the restrictions in the article.

Apple ID Name Change.        

 What to do after you change your Apple ID or password - Apple Support    I suggest doing this before you change your Apple ID or password.