Please be aware, if you bought discounted tickets they may not "bridge" the price. When we tried to add a 5th day last year (Jan 2016), they would only value them at what was paid for them. So essentially I was expected to pay the amount to bring it up to current cost + the cost to move to a 5 day. It was quite a bit more than just the cost to add a day (that the signs at all the ticket windows showed). We even went to City Hall after not having any success at the ticket booths and got the same answer.
We were very frustrated and didn't end up adding a day because the cost difference was much higher than expected. Based on ticket costs, we should have only had to pay an additional $15 per person, but they said it would be something like $45 more each, so instead of a total of $45 to add a day for all 3 of us, it would have been $135. One of them actually told me, you should make sure you purchase the number of days you think you will use if you are buying discounted tickets. We had purchased ours through Get Away Today.
I understand others have had different experiences, but I just wanted you to know what could happen.