Wait, wait, wait. Did this person purchase a three-day ticket for themselves, and now want to let you use that last day?
If that's the case, Disneyland's tickets are non-transferrable. You may be able to get away with using the ticket, but using the price of the ticket toward an annual pass would probably not work out, as the ticket was purchased in someone else's name, not yours.
On the flip side of this, it is totally possible to use the purchase of a ticket toward an annual pass. And you can purchase any annual pass that you like, the price of the ticket will just be deducted from the price of the pass.
Though some tickets don't have a name, such as when I had a 3 day Summer Nightastic pass I let 3 different people go with me on separate days. And my brother took it on the last day and made it a pass.
When you "upgrade" the ticket to an AP, just be prepared to answer a few questions that the CM might ask you such as when/where did you purchase it? When did you first use it? And when did you last use it?, etc...
And be sure to upgrade it before the end of the last day of use, otherwise it becomes useless...
They may also ask you which park did you enter on the first day of use? Did you use the magic morning and on what day?
Personally I don't see what the big deal is if a friend/family member wanted to give you their ticket to upgrade. Its like giving a gift to them or cash to them when you want to upgrade to an AP, especially if they don't want to upgrade to an AP and you do. To me why through away the money already spent? But that's just my feelings on it, everyone is entitled to their own view/opinion.
Maybe it's because I'm a rule follower and a square, but I hate it when people try to cheat the system.
I feel the same way. I don't really blame people for doing things like this, I can understand why they do. But it's against MiceChat rules to discuss something that is against Disney policy, and to hear people talk about it so openly just seems a little disheartening for some reason (especially when other MiceChatters are chiming in to let someone know HOW to beat the system).