I agree with the posters who suggested buying 1-day tickets and upgrading the ones as needed. We have done this several times with no problems.
However, about it costing the same, ticket prices go up every August (?) and I was under the impression that if you purchase your tix before that you will pay the old lower rate, however, if you are going during
free dining (after the ticket price hike) when you go to upgrade the one day tickets you will pay the new higher rate for the multiday pass. I know it's not a big difference but it is something to keep in mind.
Also, I read some where on these forums (ticket FAQ maybe?) that if you use a 1-day ticket for admission to a park before you upgrade it to a longer pass you will receive credit for the full amount that a 1-day pass currently costs, regardless of if you bought it when 1-day passes cost less (like earlier in the year or several years ago.) Theoretically if you bring an unused 1-day pass in, they will only give you credit for what you actually paid for it toward your new pass. This could help you a little, saving a couple dollars. We usually go to our first park, use the 1-day pass for admission and then before we leave that park go to guest relations and upgrade the passes.
Just remember that if you want to upgrade the 1-day pass it must be done within 14 days of the time you use it to get into a park. (If you dont use it to get into a park you can save it indefinitely and use/upgrade it in the future).
I hope that makes sense!