Hi dplan,
To address your questions in reverse order, if an email address is already associated with an Apple ID it cannot be used to change a different Apple ID account. As you indicate the account in use on your Mac and iPad use your current email, that address would be unavailable for changing other Apple IDs.
To change your iPhone account you can use the steps in Sign out of iCloud on your iPhone, iPad, iPod touch, Apple TV, or Mac to sign out of your existing account and then sign back in with your preferred account.
It's important to note that any apps installed under the original ID will not update without the entry of that Apple ID and password combination. It's generally recommended to remove any installed apps and then reinstall after you change IDs.
Be aware as well that purchases and content tied to the old account would no longer be accessible after you change. If you need to retain access to those, you may consider setting up Family Sharing whereby you can access purchases of both accounts from any device. See How to share purchases with your family for details on how that works.
Regards.