Play Pokémon Sun and Moon purchased in another country

According to Steam support (in a ticket I've sent), when you go overseas, your Steam Store billing region is automatically assigned based on the IP address you logged in using.
So naturally (for example), buying a game in Malaysia (despite having all your games bought from the proper full-paying store in Australia/USA) will impose the region restrictions when you leave the country.

Due to the way the store works, the only way to switch stores is to send a ticket to Steam support for them to change the default store that you are assigned to automatically.
(Basically - You need to ask them to make that US/Australian store the default for you regardless of the country you're in instead of the system default; the store for the country you're in).

Regarding being able to play them, games purchased at the full price (ie. from US/Australian stores) will be playable anywhere, regardless of the region restrictions, unless it is banned/restricted by law.

Deducing from that:

1. If you're a US customer buying games through the Australian store, expect all restrictions to apply, for example: Saints Row IV downgraded to a censored version and the banning of Hotline Miami 2.
2. If you're an Australian customer and had previously purchased any banned games from an overseas region, you will not be able to play these games.
   (You'll have to request to Valve for the licenses to be removed in order to re-buy it for the appropriate region).
3. If you're a US customer going to Russia, and have purchased games there - You will not be able to play them outside of Russia or any other low-paying countries.
   (You'll need to request for the deletion of the licenses in order to re-buy for the appropriate region).
4. You can still play games purchased in higher-paying regions fine.
5. Alternatively, if you do not want the restrictions, you can make a request to Steam support for them to change the default store from the system default (the region that you're currently in) to the store of your origin country (basically "of your choice").

Edit: Just recently (first week of July), Valve introduced refunds and store regions. You can change your store region if you have proof you in that country (ie. having your IP traced from that country, a form of payment addressed from that country, identity through support. etc.)

However, region restrictions still apply based on the IP address you're connected to Steam using.

Edit 2: Singapore is considered a full-paying country, Malaysia however... Isn't.

Serebii has a summary of all of the differences between the two versions.

In Pokemon Sun the first totem pokemon you encounter is Gumshoos whilst in Moon you encounter an aloan Raticate.

In Pokemon Sun you have the ability to meet and battle Kiawe whilst in Moon you can meet and battle Mallow.

You can also get different stores in the festival plaza. You can usually get them in both games to begin with but the higher level versions of them are only available in one version.

In the Battle Tree you have the ability to scout trainers. You can only scout Plumeria, Sina and Kiawe in Sun and you can only scout Guzma, Dexio, Malloww. As far as I know other characters can be scouted in both games.

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