ZeroLeftScope of this article
Everything here comes from Apple's support pages, and each claim says where it came from. Where Apple's pages are silent, this article says so and does not fill the gap. The main source is Apple's page on changing your Apple Account country or region (support.apple.com/118283).
What you must do first
Apple lists these conditions before a change. Spend any remaining Apple Account balance. Cancel any subscriptions that block a country or region change, then wait until the end of the subscription period. Wait for any memberships, pre-orders, movie rentals or Season Passes to complete. Wait for pending refunds from Apple to process. Apple also says you can cancel a pre-order.
The practical result is that a change is not instant. Subscriptions run to the end of their paid period, and only then does the block clear.
Your apps and other content
Apple states that "some types of content might not be available in your new country or region." Because of that, Apple advises you to redownload apps, music, movies, TV shows and books on the devices you will use later.
Apple's page does not list which specific apps or titles become unavailable, and it does not say your existing apps are deleted from your device. It says only that some content might not be available in the new location. To know whether a particular app is offered, check its page in the new store after the switch.
Subscriptions
Subscriptions can block the change. Apple's instruction is to cancel those that do and wait until the end of the period. Apple's subscriptions page says to open Settings on iPhone, tap your name, then tap Subscriptions, choose the subscription and tap Cancel Subscription. If the Cancel button is missing or you see an expiration message in red text, Apple says the subscription is already canceled.
After you change country and update your payment details, Apple says you can resubscribe in your new location, if the service is available there. Apple's country page also says that if you cancel Apple One or iCloud+, your iCloud storage drops to the free 5 GB plan.
Apple's cancellation page notes that you cannot cancel a family member's subscription: the family member whose Apple Account is on the receipt must cancel it. If you have a free trial and do not want it to renew, cancel at least 24 hours before it ends.
Your balance
The balance has to be spent first. Apple's balance article lists how: wait for pending refunds, then spend the remainder on App Store items or other Apple services. It also says unused balance and gift cards cannot be redeemed or returned for cash, except where the law requires. If what is left is less than the cost of a single item, one purchase where the balance covers part and a card on file pays the rest brings it to 0.
Apple's pages do not describe converting a balance into the new country's currency. They talk about clearing it before the change.
Family Sharing
Apple says that if you are in a Family Sharing group, you might not be able to change your country or region, and its country page links to instructions for leaving the group. Apple's Purchase Sharing page says family purchases are charged to the buyer's Apple Account balance first, and any remainder goes to the family organizer if Purchase Sharing is on.
That page also describes turning off Purchase Sharing: you stop sharing your purchases and lose access to the purchases made by your other members. It does not state any country requirement for Family Sharing, so this article makes no claim about one beyond the "might not be able to" line above.
Disclosure: ZeroLeft is made by the author of this site. ZeroLeft does not change any of this. It only helps spend your own balance, which Apple charges first.
Payment method for the new country
Apple's steps on iPhone are: Settings, your name, Media & Purchases, Country/Region, select the new country or region, tap Continue, review the terms and tap Agree, select a payment method and enter payment information and billing address, then tap Done. Apple notes that you might need a valid payment method for the new location. If you do not want to enter one, Apple says to wait until you are physically in the new country or region and create a new Apple Account.
On the web, Apple says to sign in at account.apple.com, choose Personal Information, then Country/Region.
What Apple does not say
Apple's pages do not say how long the change takes after your last blocker clears, whether the change can be undone, or how each individual app handles a region switch. If you need an answer on those, contact Apple Support rather than relying on forum posts.
Questions
Do I lose my apps when I change my Apple Account country?
Apple says some types of content might not be available in the new country or region and recommends redownloading apps, music, movies, TV shows and books on the devices you will use. It does not say apps are removed.
What happens to my subscriptions?
Subscriptions that block the change must be canceled and allowed to reach the end of their period. After you update payment details you can resubscribe in your new location if the service is available there.
Can I change country if I am in Family Sharing?
Apple says you might not be able to, and links to instructions for leaving the group.
What if I do not want to add a payment method?
Apple says to wait until you are physically in the new country or region, then create a new Apple Account.
What happens to my iCloud storage?
If you cancel Apple One or iCloud+, Apple says your storage drops to the free 5 GB plan.