Subscriptions - "Upselling" without expiring current service?

Hey ya'll,

We recently launched our Subscriptions (we call it Membership) and there are five different price levels, including the free trial. Yesterday we got a call from someone who signed up for the lowest paid tier for one year, about a month ago. They now want to up their membership to the top level - something hopefully everyone will want to do at some point! Unfortunately, they ran into issues, already having an "active" subscription. My temporary solution was to manually expire their service on my end and then have them sign up for the new one, but this is obviously not ideal. 

I almost posted this as a "Feature Request," but figured it has to already exist, I must just be missing something. 

Any ideas?