Here is the information from the Disneyland website:
“A Guest wishing to upgrade to a Pass from another form of eligible Disneyland® Resort theme park admission media must upgrade on the same day in which the original form of theme park admission media is valid, and must choose a Pass of equal or greater value than the retail price of the original theme park admission media when it was purchased.
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The difference in prices must be paid in full at the time of the upgrade. If an upgrade to a Pass is made from a multi-day ticket, the Pass will be backdated to begin on the first day that such ticket was used; if an upgrade to a Pass is made from a lower level Pass, the new Pass will have the same expiration date as the original Pass.
Each guest wishing to upgrade their theme park admission media to a Pass must be present at the time of the upgrade transaction. Fully used, or partially used and expired, theme park admission media, complimentary ticket media, special event ticket media, and ticket media stating its ineligibility for an upgrade may not be upgraded to a Pass. Upgrades are subject to Pass availability. Downgrades are not allowed.”
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