Hi Candice,
Welcome to the Disney Parks Moms Panel! Great idea to spread out your vacation expenses by purchasing tickets one at a time!
You can certainly do this in advance and the
Theme Park Tickets page of the website makes it super easy. The tickets do expire, but not until 14 days after you first use them. That means you can hold onto the tickets you purchase until you're ready to use them. Just be sure to only buy the number of days and options you'll need - any unused days on your tickets cannot be used after that 14-day mark has passed.
If you do buy your tickets and realize later that you'd like to make some changes, like adding days, the Park Hopper Option or the Water Park Fun & More Option, you can still do that once you're at Walt Disney World. Just stop by Guest Services at any of the them parks or Disney Springs or the concierge desk at your Walt Disney World Resort hotel.
Wishing you a magical vacation, Candice! Please visit us again if we can help answer any other questions as you make your pixie-dusted plans.
Beth