Hi there AKSpradling,
It sounds like you have two separate Apple ID accounts, and you want to change the account you currently use on your iPhone to use the email address of the other account.
It looks like you've already reviewed Change your Apple ID.
Specifically, check out the details below:
"If you see a message that the email address you want to use isn't available or is already in use, check that you or a family member aren't already using that email address with a different Apple ID. If a family member is using that email address, remove it from their account then try to use it."
Once you've updated the email address for your Apple ID, use the steps here: What to do after you change your Apple ID or password
If we've misunderstood your question, please provide additional details.
Thanks!