hotspur - "That was a bit of a ramble, but did it make sense? If it were me, I would stick to my original commitment and have a fun time at the movie. I'd tell the new friend that I would love to be there, but I already had plans to see a close friend. They will understand. Use it as an opportunity to plan another hangout with the work friend. Again, that's just me. Everyone approaches thing differently."
I think Hotspur's comment is right on target. The person who asked about this probably didnt mean it - - - but often when someone makes plans, and then later changes them for other plans - - - it can make the other person think you made contingent plans with them to 'fill your social calendar' only until 'something better came up'. This doesnt feel very good for people.They can feel devalued.
I'd carry out your original commitment to your friend and fully enjoy the evening with them.
P.S. -- I'd also let the first person know that you didnt only feel 'pressured & obligated' to go with them now. Depending on the person/situation, you might even tell them "What was I thinking ?! Going to the show with you sounds great, I'm really looking forward to it !" Given that your first friend thought you were staying longer, they might have done things to prepare for such a longer visit that they didnt even tell you about.