How often you can change the Google Play country, what happens to your balance, and which card to buy.
Google Play lets you change your country, but not often. Google allows a change about once a year, and your balance from the old country cannot be used in the new one.
Buy a Google Play card for the country your account is set to now. Google Play cards only redeem in the country they are issued for.
If you just moved, change the country first, then buy cards for the new country. Doing it the other way round can leave you with a code you cannot use.