well, you could book your package with tickets, which would be the 6 months financing.... and then at park you would take and upgrade those tickets to APs, paying the additional cost to do so.... which that additional cost will NOT get you the 6 months financing... only your package will.
But you could see when booking your package, when you add your tickets on there to add park hopper to them, add no expiration to jack up the price and however many days you want to put on there, or add on there, to jack up the price... then when you get there and upgrade, you won't have much more to pay out of pocket... and the big bulk of it will be charged to your package with the 6 months, and not as much out of pocket when you go to upgrade.
Just would make sure you can upgrade to APs if it has no expiration on it... I'm sure you can... but I would double check beforehand!
Go online and price out cost of APs and then what you want on your tickets for your package that will bring the cost up fairly close to the cost of APs.
But do NOT add water parks and more - if you want the regular annual pass that doesn't include them, then do not add water parks to your package tickets... as I believe (and can be wrong - read the ticket sticky to check)... that if you have water parks on there, then when you upgrade to APs you gotta then upgrade to the premium instead that gives you water parks, since it's already on your tickets.
