cách tính thời hạn khi nâng cấp từ Microsoft 365 Personal sang Microsoft 365 Family trên CÙNG một tài khoản Microsoft.

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Based on the documented behavior for Microsoft 365 for home subscriptions on the same Microsoft account:

1. Remaining Personal time when redeeming Family on the same account

A single Microsoft account can have only one active Microsoft 365 for home subscription (Personal or Family) at a time.

When another Microsoft 365 for home subscription is added to an account that already has one, the remaining time on the current subscription is not lost. The documentation states:

• “When you add a Microsoft 365 subscription to an account that already has a Microsoft 365 subscription, any remaining time from your current subscription is added to the new one.”

So, if a 12‑month Microsoft 365 Family subscription is redeemed in 03/2026 on the same account that has an active Microsoft 365 Personal subscription (expiring 08/2026), the remaining Personal time is added (after conversion) to the new Family subscription. As a result, the Family subscription can be longer than 12 months, subject to the 5‑year maximum total duration.

2. Proration / conversion of remaining Personal time

The documentation confirms that when a subscription of a different type is added, the remaining time is converted based on the relative value of the plans. Example given:

• “If you use a Microsoft 365 Personal product key to renew, and you currently have a Microsoft 365 Family subscription, you'll be offered a choice to stay with Microsoft 365 Family subscription and receive less than a year of subscription benefits, since Microsoft 365 Family costs more, or you can switch to Microsoft 365 Personal.”

This shows:

• Remaining time is not simply added 1:1 when switching between Personal and Family.
• Instead, Microsoft applies an internal proration/conversion based on plan prices/values.
• The exact numerical conversion ratio or formula is not specified in the provided documentation.

The only documented rules are:

• Remaining time “is added to the new one.”
• Remaining subscription time “can be extended.”
• Total combined term cannot exceed 5 years.

3. Duration for shared Family members

For switching from Personal to Family, the documentation states:

• For Family: “Subscription sharing: Share subscription benefits.”
• “Remaining subscription time: Can be extended.”
• “Whether you stay with the same subscription or switch to a different subscription, your online storage capacity per user doesn't change. For example, if you have only one user on the subscription, you'll have 1 terabyte of storage, but if you have Microsoft 365 Family and share with five other people, you get a total of 6 terabytes of storage.”

This reflects that:

• There is a single underlying Microsoft 365 Family subscription owned by the primary account.
• Other members receive shared benefits from that one subscription.
• There is no separate 12‑month clock per member.

Therefore, all members added to the Family subscription keep their access only until the Family subscription’s overall expiry date. When the Family subscription ends, all shared members lose the Family benefits at the same time as the owner.

4. Principle for the specific scenario (Personal to 08/2026, redeem Family 12 months in 03/2026)

From the documented rules:

• “When you add a Microsoft 365 subscription to an account that already has a Microsoft 365 subscription, any remaining time from your current subscription is added to the new one.”
• “If you already have a Microsoft 365 Family or Microsoft 365 Personal subscription, the table below summarizes your options when you switch from one subscription to the other.”
• For both directions (Personal → Family and Family → Personal): “Remaining subscription time: Can be extended.”
• “You can extend the duration of a Microsoft 365 subscription for up to 5 years.”

Applied to the example:

• Current state: Microsoft 365 Personal valid until 08/2026.
• In 03/2026, a 12‑month Microsoft 365 Family subscription is redeemed on the same Microsoft account.

Result according to the documented principles:

• The account switches to Microsoft 365 Family in 03/2026.
• The 12‑month Family term is applied from the redemption date.
• The remaining Personal time (from 03/2026 to 08/2026) is converted according to Microsoft’s internal proration and added to the Family subscription.
• The final Family expiry date will therefore be later than 03/2027, but the exact date depends on the internal conversion ratio, which is not detailed in the documentation.
• All shared members added from 03/2026 will have access until that final Family expiry date.

To see the exact end date for a specific account and key, the documented guidance is to manage the subscription at https://account.microsoft.com/services, where the updated expiry date is displayed after redemption.

References:

• What happens if I add another Microsoft 365 for home subscription to my account?
• How can I upgrade from a Microsoft 365 Personal to Family subscription?
• Microsoft 365 Family