Here is what we encountered on our visit in November. We needed to add days. My daughter was on the last day of her ticket and my husband and I still had several days left on ours. Now, this could have been the CM we talked to. He was at the ticket booth entering Epcot from the International Gateway. I must say he was perhaps the worst CM we have ever encountered. He said because she was on the last day of her ticket and it had already been scanned that day, he could not see how much she originally paid so he made us pay the regular price for a 1 day admission. He did include the park hopper as he could see she already had it. Now, for my husband and myself, he added the day at the @1$0 rate. There was a line of people behind us and it didn't matter how much I said to the CM, he wasn't budging. My family was pretty embarrassed and I decided it wasn't worth it. I just paid the money and went on into the park.
That is just to say, if you want to add days, don't wait until the last one.