title: Stripe subscription upgrading and downgrading.
body: <p>This question is for anyone who's experienced with Stripe. I've run into a situation that may be pretty specific, but I'm hoping I could get some help with. I am creating a system where a user can pick from many different options and set up a yearly subscription based on their selections. So there is no set plan cost, the subscription cost is variable. </p>
  
  <p>Now, if someone creates a subscription for $100.00 dollars, then in a month decides next year they'd like to reduce their subscription to $50.00 they can do so. However, they will not receive a prorated refund due to the downgrade for the current subscription period. My issue is, let's say they change their mind and realize the $100.00 service is worth it. So much so they actually want to change their subscription from $50.00 to $120.00 in the current subscription year. I would like to charge them a prorated fee, but if I do that they will be charged for the difference of $50.00 and $120.00, not $100 (which they paid originally) and $120. Is there anything I can do to allow for such a situation to occur? </p>