I've had a similar situation 2 years ago. I bought tickets through UT and linked them on the app.
after that i was able to change the date on the app directly. however i realized i made a mistake in the date when i changed it. So I tried to change them again but this time it would not let me.
I called universal guest services and they said they cannot do it for me on the phone since i purchased the ticket from a third party, but they said on-site guest services would be able to do that once im there. and surely once i got there (at that time, the tickets were unused but expired) on the same date i wanted to visit the park, i stopped at the city walk guest services who were able to change the date and i had to pay a small difference of $5.
I realize it was a gamble, but call guest services and see what they tell you.
I do not have any experience with returning ticket to UT as I have not attempted that before.