If I wanted to modify an existing payment profile that is not being used in order to change it to another payment provider that is not listed, is this possible and if so what would need to be changed?
You are better off creating a new payment profile for the payment processor rather than modify an unused one. XenForo supports an unlimited number of payment profiles, so there's no reason to not have a new one. You can of course start with the code from an existing one to help you along.
All you need to do is tell XenForo that there is a new payment profile available by adding a record to the xf_payment_provider table. In there, you tell XenForo what the class is for your new payment provider(s). As an example, I have 2 additional payment providers (one for Bitcoin and one for Monero). My xf_payment_provider table looks like this:
provider_id
provider_class
addon_id
bitcoin
DigitalPoint\Crypto:Bitcoin
DigitalPoint/Crypto
braintree
XF:Braintree
XF
monero
DigitalPoint\Crypto:Monero
DigitalPoint/Crypto
paypal
XF:PayPal
XF
stripe
XF:Stripe
XF
twocheckout
XF:TwoCheckout
XF
...anyway, long story short is you can add your own payment provider, you don't need to modify an existing/unused one.
Teaching you how to code an addon is really outside the scope of this thread. Ask around to n the developer forum and I’m sure you can find some help. I was strictly just saying that you don’t need to override/modify existing (but unused) payment providers because XenForo doesn’t have a limit on how many you can have.