How do I change Microsoft gift card balance currency?

Well no as cards are region locked

So buying a CAD gift.card you get cad balance consoles aren't banks

Hello MartinHero!

Are the gift cards you have show "Microsoft Points" instead of your local currency? These types of gift cards have been discontinued years ago, and have moved to local currencies since.

This move from MSP (Microsoft Points) have then since enabled the sharing of your Microsoft Account Balance to be used on your Xbox One, and Windows purchases. As of now, it isn't possible to purchase Xbox 360 games young Microsoft Account Balance.

edit: No, you do not need Xbox Live Gold to redeem any currency-based gift cards.

That is actually outdated information. It is referring to Microsoft points which were discontinued in 2013. In order to pay for a gamertag change you would need a credit card, Paypal, or you would need to add currency to your Microsoft account balance with
a Xbox gift card.

stored value

What happens if I buy a Microsoft gift card in Germany in euros and add it to my account, and then move to the United Kingdom? Does it convert from euros to British pounds?

Balances are not converted from one currency to another. You must spend the value of the currency in which it was purchased. You will have to buy another Microsoft gift card when you move to another country or region, because the balance you have in one
currency will not work in another marketplace.

Hello ARTURGRZEŚKOWIAK, this is Manuela. I'm an Independent Advisor and community user like you, ready to assist with this.
Not a Microsoft employee.

When you redeem a gift card that is from a different region, you're going to see multiple currencies for your Microsoft balance.

In situations like that, I'm afraid to say that Microsoft support is not able to convert currency or change currency from one country to others.