Last year I had eight full days but for some reason only bought a 7 day park hopper ticket. They upgraded it at the hotel when I checked in and It only cost me $8 to upgrade 2 adult and 2 child tickets. This is what it would have costed had I originally booked it that way. I believe you can upgrade (add on) to your tickets at any time and get charged what ever it would have been had you originally booked it that way. (without being charged like it was a whole new ticket) Does that make sense?
I believe it's only like $50 to upgrade to park hopper option for each ticket and just a few dollars to add extra days depending on how many days you originally have. I have heard of people also upgrading to annual pass and only paying the difference between that and what they paid for their original tickets.
Actually we ended up adding an extra day to our vaction in Nov/Dec because we used a discount code after I had paid for the trip and they couldn't refund me the difference because I no longer have the credit card I used to book the trip. So we scheduled another night at the hotel and dining plan (for $19 more) but DH forgot to add another park day. The upgrade is only $4 for all four tickets because I'm going from 9 days to 10.
hope this helps!