Not only is it that simple, but you can later change the configuration back to a pistol or to a long barreled rifle and later back to a SBR at your pleasure. Similarly, you can change the caliber and/or barrel length at any time. Although the ATF requests that you notify them of configuration changes, there is no law or rule saying that you have to notify the ATF.
While the AR is in either the Pistol or Rifle configuration it is not subject to the NFA rules and can be transported across state lines without getting prior permission. This is useful when you want to hunt or shoot in a state that does not allow SBRs. Be certain that you follow the rules of whatever state you are in and also don't reconfigure it to a SBR in another state unless you have an approved Form 20. When changing calibers, I believe the correct caliber should be marked on the barrel, upper receiver, or lower receiver so that it can be clearly seen. I am not 100% certain that is a legal requirement, but seems reasonable and is easy.
You may even sell your AR without any transfer tax if it is not in the SBR configuration. I would consider that a waste of a $200 tax, but it is permissible.