Hi Yibo,
About this question, I suggest you contact our sales and billing team for better service. They can answer this subscription related question more properly and professionally. For detailed information you can see: office.microsoft.com/.../contact-support-for-technical-presales-billing-and-subscription-issues-HA103836042.aspx
Anyway, you can also directly contact our sales and billing team via phone to consult this question. You can find the phone support number from this link: virtualchat.support.microsoft.com/.../default.aspx
On this page, please select your country or region to get the phone number.
Thanks,
Young Yang