Freestyle has nothing to do with whether there’s a class plan. It has to do with whether the instructor has created their own class plan or whether the instructor has purchased a license to a format like Insanity or RIPPED etc. where the plan and choreography are provided.
It requires different skills for the instructor. Freestyle means coming up wih a class framework and determining the best way to implement it, and modifying as necessary for individuals who need options. Pre-choreo is to memorize and repeat someone else’s work as they intended it.
Well taught, it shouldn’t feel much different for the participant, other than that class plans will usually change more often for freestyle classes than for prechoreographed formats.