This link used to have this information but it no longer contains this useful tip:
If you can’t change your iTunes Store country or region - Apple Support - https://support.apple.com/HT204447 - "If your store credit is less than the cost of a single item on the iTunes Store, App Store, or iBooks Store, you can add a credit card to your account [https://support.apple.com/kb/HT201266]. When you buy an item, you will use the remaining store credit and charge the remainder of the purchase to your credit card."
"If you don't have a credit card, contact iTunes Store support." Apple will remove the balance and keep it.
iTunes Customer Service Contact - https://www.apple.com/emea/support/itunes/contact.html
I haven't heard of anybody trying this but: Create and add money to an iTunes Pass - https://support.apple.com/HT203021 - "With an iTunes Pass, you can add funds to your iTunes account at Apple Stores or select retailers. You can use the funds to buy music, apps, movies, books, or TV shows*." Limited countries only. Add exactly what you need to buy something (don't forget tax, if applicable).
You should only be doing this change as part of truly moving to a new country. Some people just want to temporarily use the store in another country for access to certain materials but that is not allowed.