When I want to change the payment method to another country, I get an error that says something like this:
AN ERROR OCCURRED
Oops! Looks like something went wrong. Please go back and try again.
If you keep getting this error contact customer support to get further help.
Can confirm, tried switching from USA to Czech republic with the same result - tried in Edge and Chrome browsers as well as in incognito mode.
Noticed Germany does not work as well, but United Kingdom works. I was doing this when purchasing gift code and since the code is (according to purchase page) country-locked this is quite a major issue, since it makes Minecraft unpurchasable in certain countries...