If you can get a refund it's not a sunk cost. Usually you cannot get a refund and anybody who refunds a movie because you didn't like it is sort of a moron. Movie theaters don't guarantee that all the movies they show will be good.
The fact that you can watch another movie is the same as the fact that you can go to sleep or leave. It doesn't change the fact that the movie ticket is a sunk cost. It just illustrates the concept of basing economic decisions on future costs rather than sunk costs, i.e., don't stay in this movie just because you paid for it, that just increases the loss on your investment.
To make my point especially clear, even if you buy a ticket for The Bounty Hunter, but then you realize your mistake early enough to change theaters and watch How to Train Your Dragon, and you leave the theater perfectly ecstatic about your moviegoing experience, the ticket was still a sunk cost all along. Or even if you never even went to The Bounty Hunter, but rather walked up, bought a ticket for HtTYD, watched the movie, loved it, left, the ticket was STILL a sunk cost.