Member not being removed from old subscription upon Upgrade

Ran into an interesting email from a customer today.

She had previously been a "Sentinel" level member on our site, and chose to upgrade to a "Champion" level member.

She was successfully added to the "Champion" level, but Membership plugin did not remove her from "Sentinel" level, which means she was being charged for both memberships.

I am sure this is a simple oversight on my part, but seeing as how Membership is quite a complex plugin, I thought I would seek out wisdom from those with more knowledge on it than I.

Thanks for your help, as always.