Problem configuring plan to allow back-end upgrades to it

I believe so - plan params attached.

Are both plans configured as exclusive? If so it's an upgrade from 1 plan to another. So upgrade conditions will apply. Conditions like only allowing upgrade if it's more expensive. Is this plan lower price than the other plan?

Both plans are configured as exclusive. Both plans do not have the restriction of upgrading only if it is more expensive.

The puzzling thing is that there are two members with the same active subscription, and it displays for one but not the other as an upgrade option (both are visible in backend only).
The only difference I can see between the users is that the one where it is not displayed has either an expired or unsubscribed subscription for the plan which is not displaying.
If I get rid of that old subscription in MySQL, it then works.

Strange, was the plan changed after that user had already subscribed? Stuff like it having a parent plan at some point and now doesn't? Sometimes that can cause visibility issues.

We never had a plan hierarchy, so no, that hasn't changed. I looked at the expired/unsubscribed subscriptions, and both have replaces subscription = 0. Is that expected?

Anyway, what I'm really interested in is how to deal with this when it happens again. I can go in and delete the subscription in MySQL, but the people administering the subscriptions on the back end can't. Is there an easy way for them to fix it in the back end?

If you still have a user where this issue can be seen I can take a look if you PM backend super user login credentials. My best guess is still some configuration problem somewhere.