Here are two additional methods that Apple does not include in the support article Change your Apple ID country or region - Apple Support . In the interest of full disclosure I am posting them separately.
+ "Spend any balance remaining on your Apple ID"
If you then find your store credit is less than the cost of a single item and you still have a valid payment method for your old country such as a credit card, you can use the payment method to make up the difference in the cost of an item ( How apps, content, and subscriptions from Apple are billed - Apple Support ). Perhaps buy something such as a music track which you can download and also keep a backup copy. Remember that anything purchased may not be available for re-download in another country but as long as you have your own copy you will always have it.
This method, where you get to use your balance by spending it instead of giving it to Apple, used to be in Apple's support article but a few years ago Apple stopped mentioning it even though it is still perfectly effective.
+ "Check your Apple Account balance" refers you to Check your Apple Account balance - Apple Support
Note the sections "If you have an Apple Account balance that you can't use", and "If you plan to delete your Apple ID or change your country or region" Again it is valid and is even in Apple's documentation. However, since it only works in limited situations, it may be this is way it is not really recognized by Apple as a practical way for removing account balance, though it may work for some users.
