Hey bright33,
If I understand this correctly, you purchased an iPhone in the US and you moved to another country. Now you want to change the country because for your payment option. Is that right? If so, then I believe I can help you.
It sounds like you're looking to Change your Apple ID country or region. There are some requirements that you must meet before you can change the country or region for your Apple ID. Follow the steps from the article I provided you and let me know if it helps.
If you continue to have issues, you'll need to contact Apple Support team for further assistance: Contact Apple for support and service
Thanks for posting in Apple Support Communities and have a great day!