Thank you for all the information you have provided.
We received the same response from VRChat management.
The contents are as follows
Hello,
It is not currently possible to upgrade your subscription from a monthly subscription to an annual subscription. However, as a one-time exception for this particular sale, we are willing to manually grant you the extra months if you provide us with proof of purchase (a screenshot of your purchase receipt should suffice) of a VRChat+ Yearly subscription within a week after your current monthly subscription period expires.
One
VRChat Team
It is nice to know that we can get an individual response from VRChat management.
If you would like an annual subscription, please cancel your current monthly subscription, wait for it to expire, sign up for an annual plan, and then contact VRChat.
My reply will close this forum posting.
Thank you very much.