Be careful with park Hopper it isn't what it used to be. Currently due to Covid you can't hop until after 2pm and just the other day they limited hopping to the Magic Kingdom to 2-4pm only if there is room to reduce crowd size to watch the sub-par fireworks show. They need to bring back the last show it was far superior.
As listed above, it can be purchased with your Disney Vacation package as an add on. If you did stand alone tickets, you can have them upgraded prior to traveling or at guest services outside the entrances of the four theme parks. Many things are changing often right now, but Park Hopper begins at 2 pm for any parks that are not at full capacity. You must have checked into your first park of the day that you had reserved prior to trying to Park Hop.
It's an add on to your park ticket. You can buy it at purchase time or upgrade at any ticket counter.
Add on purchase
Purchase online or at a park
It is an add option when you get your ticket. Or you can check for ability to upgrade.
When you buy a ticket you can choose a hopper ticket.
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