Microsoft 365 features that help users manage their subscriptions, account settings, and billing information.
Hello,
Changing the country in your Microsoft account profile or updating the billing address will not convert an existing subscription from Vietnam to Singapore.
Also, cancelling the subscription usually only turns off recurring billing. The Microsoft 365 Family subscription remains active until its expiration date. Microsoft states that you cannot purchase a subscription for a new region until the existing subscription from the previous region has fully expired. https://support.microsoft.com/en-us/accounts-billing/change-your-country-or-region-in-microsoft-store