Hi, Vicki!
You have a few ticket options available:
Option #1: Do as you plan. Use up your one day ticket, then buy a new ticket for your remaining days. Tickets can be purchased at any theme park ticket window or right at your resort.
Option #2: Buy your new Park Hopper ahead of time to save you some time once you arrive. You can do this online at
www.disneyworld.com and have the tickets mailed to you.
Option #3: Buy your tickets ahead of time at your local AAA office or online at UndercoverTourist.com. Both will save you a few dollars on tickets.
Option #4: When you arrive in WDW, take your existing ticket to your Resort Concierge or a Guest Relations desk. They can take the value of your current ticket and just add it to a new ticket so you are all-in-one!
Any of these options will work and get you what you need. Just decide what will be the easiest method for you. Good luck!