Thank you for your reply. Yes, the behavior seems unexpected. Attached is a screen shot of all of my Plans. (Note that "Staff" and "Neighbor" at the bottom are lightly used and/or ready for retirement.)
I have confirmed that both "Sponsor" and "Gold Sponsor" plans have the following setting:
Edit Subscription Plan > Presentation tab > General settings > "Exclusive plan" value is:
"Exclusive, other plans of same parent cannte be subscribed same time, but one subscription is mandatory"
I am quite sure that both plans were configured this way before the member's upgrade from "Sponsor" to "Gold Sponsor."
Come to think of it, it doesn't look like the prorated amount remaining from the "Sponsor" subscription was applied to discount the new "Gold Sponsor" subscription. (I thought I had configured it to do so.)
I had made and saved some changes to both Plans right around the time of this signup ... however, I hadn't touched the "Exclusive plan" value, which was set this way during the upgrade.