In any case, by buying a ticket for a movie but seeing another one, you’re denying the movie’s production company the income which is rightly theirs (unless, I suppose, the ticket you bought was for a movie produced by precisely the same company, which is unlikely, given the complicated way in which individual movies are financed, and the numbers of different directions ticket income would be split).
I would say that you’re obtaining an advantage that you’re not entitled to. You might have paid for something you didn’t, in the end, consume but you’re still consuming something you haven’t paid for. And you’re depriving the rightful recipients of the money due from the ticket you should have bought. And, guess what, that’s theft. It may be a small amount, but it’s an amount nonetheless. There may be no net benefit or disadvantage to your pocket book, but the effects go beyond your own finances.
Basically, we’re commanded to love others. Is it loving to deprive someone of something to which they’re entitled, no matter how insignificant in the grand scheme of things, just because we feel like it? I’d say no. I’d also say that to do so is sinful, if we’re aware of the full facts. (And if you weren’t before, you are aware now!)