I'm going to use a changing room even though other people's stuff in there.
Why would you use someone else's changing room?
Because they are not using it at all.
Even if someone else isn't using their changing room at the moment, you using it instead is not appropriate. Stay out of other people's private spaces.
It'll be fine. I'm bored and don't have anything better to do.
They might be back any second and will be very upset if they find you in there with their things. I'm sure if you wait just a few minutes, a different changing room will become available for you to use.