What happens to my existing subscription if I upgrade?

My existing subscription just renewed in August 2017 so I have 7 more months left on that subscription.  If I upgrade today, what happens to the 7 months I've already paid for?  Thanks!

The amount you have payed will be added to the final price of your upgrade. If you are having issues while upgrading please let us know we can refund the 7 months that you didn't use from your existing plan.