I was hoping there was a number because I only want to buy the number of days we will need. Thanks !
Don't know if it'll really help, but I think WDW will allow you to add days to your new ticket, especially if it's unusued.
I have no idea how long you're staying at WDW. Let's say you're planning on staying at WDW for 6 days, and you think your old ticket has 2 days left over. You can buy a new 4-day ticket before you go. Once you arrive, you go to Guest Services with your old ticket, and there isn't 2 days left over on it. There's only one day, or no days at all. At that point, you can upgrade your new 4 day ticket for a 6 day ticket.
If I'm wrong, someone please tell me.
Bear in mind, once your ticket is for a longer stay than 3 days, the prices drop significantly:
3 Day MYW Base $204.48
4 Day MYW Base $215.13
5 Day MYW Base $219.39
6 Day MYW Base $221.52
7 Day MYW Base $223.65
8 Day MYW Base $225.78
9 Day MYW Base $227.91
10 Day MYW Base $230.04
The difference between staying for 3 days or staying for TEN DAYS is only 15 dollars. Of course, that's just for a base ticket, no park-hopping. But the difference between a 3 day and 10 day ticket with park-hopping and water parks and Pleasure Island is less than 30 dollars ($305.66 versus $331.22). The difference in price only becomes sizable if you add the "no expiration" option to the ticket. So depending on how long you plan on staying, it might make sense to pay for a ticket that is good for your entire stay. But bring your old ticket anyway, and find out if it still has any valid days on it. You can always use it on your next trip. Maybe one day you'll do a weekend jaunt. Chances are, you'll spend so little on adding extra days to your new ticket, even on a tight budget, it could be as little of a difference as one less beer a day, or a single counter service versus table service meal.
Here's a link to all of Disney's pricing.
http://www.allearsnet.com/pl/ticketchart.htm