You can make the upgrades to your 6 day tickets within 14 days of first use. If you go past 14 days, you will not be able to add non-expiring or extra days. Your tickets will expire after 14 days of first use unless you get the non-expiring add on. As PP said, 10 days is the max you can have. You can only add 4 days to your six day tix.
If you make them non-expiring, then I would rec. you add the water parks and more option. At $53.25 (inc. tax) it's a great value. If you have a 10 tix, you'll get 10 plusses-that's just over $5 per visit. So worth it.
Here's the numbers (all inc. tax., adult)
Your 6 day base ticket cost $231.11
Add on Non-expiring $69.23/6 day
191.70/10 day
Add on Park Hopper $47.93/no matter how many days
Add on Water Parks and more $53.25/no matter how many days
So, if you want to upgrade to a non-expiring, 10 day, PH, water parks and more the extra cost would be $301.39/adult. (532.50 less 231.11)
A 6 day, non-exp, PH, waterparks plus, would be an additional $170.41 ($532.50 less 231.11) The difference between 6 and 10 days is $130.09 for 4 extra non-expiring days and 4 more days for waterparks and more.
We always buy 10 day premiums (non exp., PH, waterparks plus) and bring them back for a few trips. We have alot of waterpark plusses stack up right now.
If you don't want to go the the waterparks, etc. you would save $53.25 per tix. No Park Hopping would save $47.93.
A 10 day/non-expiring, non-PH, non-waterparks and more would cost $431.43, a difference of $200 more than you paid for your 6 day base tickets.
Hope this helps.