if it works like everything else, if you upgrade, I believe they would pro-rate the upgrade, I think.

I know when I added another retainer for instance, they did not have one retainer that extended 15 days past the rest, they just charged for 15 days that month they were added, and full rate to bring them in line. I think subs work the same, you are charged for whatever percentage of the month that remains. so your expiry remains the same.