You should check the department guidelines/rules on when students pick their thesis advisor. At most schools in the US, this doesn't happen until a little while into the program. At some places, you are even encouraged to work with multiple professors at first and pick one later. If you can't find a clear answer, definitely talk to the prof in charge of grad students in this department to get clarity on the advisor picking process. Do this before agreeing to work on any projects.
What do you mean by the last part ("Is it Ok for me as a TA to change my mind and turn to experimental while the theorist advisor recommended me for this position?"). Are you saying that the theoretical CMP professor in this department recommended you for admission? If so, then you should check whether your admission offer is dependent on you working for this professor. Some schools do admit students directly to a professor. However, many US schools do not do this so even if one of the profs was interested in you initially and made a recommendation for you to be admitted, if there is another prof that is interested in working with you and you're interested in them, then that should be okay. Again, check with the prof in charge of grad students on whether you are restricted to certain profs as advisors.
On the other hand, if by "theorist advisor", you mean an undergraduate advisor who wrote a letter of recommendation for your application, then this doesn't matter at all. Once you start the grad program, everything from your application is basically ignored and you are starting fresh. It doesn't matter what you said you wanted to study / work on in your SOP, all it matters is what happens from here on. So, if your interests change after starting your program, feel free to make changes to your plan as long as they are within the program's rules/policies. You're not obligated to do anything you said in your SOP.