The way I understand it, unless you are changing calibers as a business, for a customer, you have nothing to worry about. SOT/02/07 licensing pertains to that aspect. FFL 01's are not licensed to make or manufacture for sale any firearms....02's and 07's are different, and fees are much higher for those licenses, as well as corresponding paperwork requirements. There are numerous caliber conversion kits available for AR platforms, as well as many other type firearms......no special license required to change your own weapons.....
At least that is the way I understand it, and have been living by that philosophy for 30+ years......do correct me if I deserve to be in prison.....:thinking: