The movie was inspired by the story of Tim Owens after The New York Times writer Andrew C. Revkin wrote a story on him. Warner Bros. decided to make a movie out of it and, as happens so often when studios try to make a movie about a real life person or a band, that person wants to make sure they can control how they are portrayed as to not destroy their image. But studios are not keen on handing over that kind of control over their projects. According to MTV News,
Initially, Judas Priest were interested in the project, and Warner Bros. wanted the band to write music for the soundtrack. However, the group wanted creative input on the film, and that soured the deal, said Revkin, who was hired as a creative consultant for the movie.
So at that point they turn it into a fictional story to cut off any lawsuits. That also frees them from having to be factually correct and giving them freedom to make the story more interesting, like adding Jennifer Anniston as a love interest. Tim Owens said, “They fabricated things and decided to pull away from my story and make their own because I guess mine was too normal."
The writer they hired to write the script said this:
I pitched my take on the story – an obsessed twentysomething selling office supplies in a rust-belt town has completely subsumed his own identity while fronting a tribute band. It’s only after this Überfan becomes the lead singer for the band he worships that he discovers his own identity and individual voice.
That wording makes me think it was intended to be a fictionalized version from the point he became involved. Although he did a lot of research, even going to Owens' home town, to get the character right, he did not mention speaking to Owens himself or any other member of the band. The fact that the character in the movie ends up being replaced by the original singer was completely fictional, but prophetic, as Owens left Judas Priest in 2003 to be replaced by Rob Halford, two years after Rock Star was released.