Is there any way I can have access to both app stores (I have a Canadian credit card and an American bank credit card as well). Or do I have to choose one or the other.
second question - songs downloaded directly through iTunes Canada - not a subscription. Will those still be mine if switching to US?
The sections below are copied from the USA Apple Media Services Terms and Conditions document but I suspect they are very similar for all versions of this document --> https://www.apple.com/legal/internet-services/itunes/us/terms.html - Last Updated: September 16, 2020
"Our Services are available for your use in your country of residence (“Home Country”). By creating an account for use of the Services in a particular country or territory you are specifying it as your Home Country."
"Terms found in this Agreement that relate to Services, Content types, features or functionality not available in your Home Country are not applicable to you unless and until they become available to you. To see the Content types available to you in your Home Country, go to the Services or visit https://support.apple.com/en-us/HT204411. Certain Services and Content available to you in your Home Country may not be available to you when traveling outside of your Home Country."
I know that isn't directly answering your question but I am not a lawyer so I will have to leave it up to you to interpret it as you will. Much earlier versions of this document used to say: "The iTunes Service is available to you only in the United States, its territories, and possessions. You agree not to use or attempt to use the iTunes Service from outside these locations. Apple may use technologies to verify your compliance."
"Terms found in this Agreement that relate to Services, Content types, features or functionality not available in your Home Country are not applicable to you unless and until they become available to you. To see the Content types available to you in your Home Country, go to the Services or visit https://support.apple.com/en-us/HT204411. Certain Services and Content available to you in your Home Country may not be available to you when traveling outside of your Home Country."
This article describes how to change your country setting and the requirements and results of doing this:
Read how to "Change your Apple ID country or region" click here --> https://support.apple.com/HT201389
"Redownload your apps, music, movies, TV shows, and books on any devices that you might use to play or read them in the future. That's because some types of content might not be available in your new country or region. (Learn what types of content and services are available in your new country or region. - https://support.apple.com/HT20441 )"
"To make sure that you can access your previous purchases in your new country or region, redownload your previous purchases on every device on which you might want to view the purchase before you change your country or region." Subscription services have to be canceled and started again.
Before you change country settings, download anything you want to keep and alsm make backup copies. There's no gurarantee they will still be accessible in your new country.