If you call, Disney will tell you that party tickets are transferable until the moment they've been used to enter the MK! On your
MDE, you'll see that they can be reassigned to any other guest who is on your MDE or with whom you are friends.
The trick is that once you have linked a plastic ticket to yourself on your MDE, that physical card is linked to you forever, just as any of your magic bands is. So you can reassign the ticket entitlement, but not the plastic ticket it may have been issued to you on.
If you haven't linked the plastic card to the MDE, then reassigning is easy. And once reassigned, you can then link the physical ticket to whomever is going to actually use the party entitlement.
If you have already linked the plastic card, then that card is linked to you. You have to reassign the party ticket to someone who already has a magic band that they can use to access the party entitlement to enter the MK. It can definitely be tricky. Having the plastic card in hand is not all you need -- if the party entitlement has been reassigned to someone else, you will not be able to enter the parks with the plastic card that once had the party ticket on it!