Welcome to planDisney, Greg! I'm so glad to be able to answer your question today. Based on what you've asked, I think you could be talking about one of two things: either adding the
Park Hopper Option to a portion of your ticket, or having a ticket length that is different from your length of stay at a
Disney Resort Hotel. I'll go over both answers for you, so let's jump right in!
At Walt Disney World, the Park Hopper Option is an add-on for your whole ticket, not something you can turn on for just one day of a
multi-day ticket. You can absolutely add Park Hopper anytime before the last day of your ticket, but when you do, it applies to the entire remaining length of that ticket. So if you hold a 5-day ticket and decide on day 3 that hopping would be helpful, you can upgrade that day, but the Park Hopper will be active for days 3, 4, and 5.
Here's the money-saving twist: the price to add Park Hopper is the same whether you add it on day one or wait until you only have one day left. Because the cost doesn't go down as your trip goes on, you'll get more value by upgrading as early as possible if you think you'll hop even once. If you're unsure you'll use it at all, you can wait until your plans firm up, just know you'll pay the same either way, so earlier is usually better for maximizing use.
Now, onto that second potential answer! If your question is about ticket length vs. resort stay, those don't have to match. A 4-night Resort Hotel stay doesn't require a 4-day ticket. You can buy fewer park days, or none at all, depending on your plans! You do not need tickets to stay at a Disney Resort Hotel. I actually like having a stay that's a little bit longer than my ticket length! That way, I have some free days to explore the resort, relax by the pool, and enjoy the amenities of where I'm staying.
I hope this answers your question, Greg, and please let us know if there's anything else we can do to help you plan your magical vacation!
Jessica