Before you change country make sure you have downloaded and backed up any purchases from your previous country's store. You will not, in most cases, be able to re-download them again once you change countries and it won't be easy to change countries to the previous one once you lose your payment method for the former country (as well as a wait period). As long as you have them on your computer you will be okay.
Read this Feb. 2018 post by Dah.veed post regarding possibly still having access to some apps if they are in the stores of both countries - https://discussions.apple.com/thread/8289669?answerId=33055125022#33055125022
Change your iTunes Store country or region - https://support.apple.com/HT201389 - In small, faint print, "You might not be able to redownload some of your purchases from your original country or region." This may also mean any subscription services you use may no longer work (e.g., Apple Music).
Check the details in the article I link because there can be reasons such as not having correct payment type, an ongoing subscription, or still having a balance in your account that will prevent you from changing. To zero your balance:
Option#1:
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."
Option #2:
"If you don't have a credit card, contact iTunes Store support." Apple will remove the balance and keep it.
iTunes Store support. https://expresslane.apple.com/GetproductgroupList.do?&PRKEYS=131629
or
iTunes Customer Service Contact - https://www.apple.com/emea/support/itunes/contact.html
Option #3
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.