In the Scooby Doo movie, Thelma, was wearing an orange sweater in most of the movie. Not to give it away, when at a certain part where she had no way of holding on to the sweater or changing, she ended up in an orange shirt, then later back in the sweater.
I noticed this too, but this is at the part in the movie in which everyone is posessed by the creatures. They are trying to show how everyone has changed by changing their clothes, attitudes, and personalities. They must have decided that the sweater was to dorky for the new "cool" Velma. The same goes in the deleted scenes when she's dancing in the locker room in a bikini.
PS:It's VELMA, not Thelma. Why can no one get that right?
She actually did change her shirt in a deleted scene on the DVD. When she gets posessed by one of the demonsit goes into the locker room and does this weird dance number when Daphne walks in and catches her, the demons then freaks out and captures everyone in the locker room.
First of all her name is Velma, not Thelma. Secondly, there is a reason she no longer is wearing the sweater and that would be because she was inhabited by the monster. The monsters took over the humans and did with them what they pleased. Apparently, they wanted Velma in that shirt.