How do I get icloud "Change Storage Plan" to work?

My free 20 Gig first year storage ends on Sept 30 2013. I want to buy that same plan. However, on my mac, when I go to the apple menu/preferences/icloud/manage/change storage plan - the 20 GB plan is highlighted but the "next" button in that window is not highlighted so i can't proceed to the next window to buy the 20 GB plan. It lets me do it if I want to buy the 50GB plan. What do I do? I don't want to let the current plan expire as it will dump me back to the free 5GB plan and I will loose what I have already stored (above the 5GB).