Problem configuring plan to allow back-end upgrades to it

I have two plans that seem to be configured identically under workflows. When I go to user's membership on the back-end, one of them shows as an upgrade option, but the other one does not.
Where else should I look for param differences?

Edit your plan that's not visible then under the Workflows tab review the parameters under the "Subscriptions upgrades workflows" section as upgrades are entirely controlled from there.

I went through and carefully compared the params for the missing plan and one that is displayed, and I can't find any differences.
Any info I can send that would be useful?

Is the plan accessible? It may be an access permission issue and not an upgrade permission issue. Under the Access tab ensure you've set your parameters correctly as it will check access against the profile you're editing not against yourself. The idea is you can only assign the user to plans they can actually access except for the "backend only" plan setting.

Plan is accessible.
I discovered that two users with identical subscriptions showed different behavior: plan was displayed for one but not the other.
Digging deeper,I found that the one where it was not displayed had an expired subscription of the same plan. Deleting that expired subscription fixed the display problem.
Clearly not the right way to solve it, though.

Just to be clear, the expired subscription was to the plan that was not being displayed.

Does the plan allow renewals then? Backend will give the option to renew plans as well, but it again respects whatever the plans configuration is. So if it didn't allow renewals it wouldn't show there for renewal.