There are a number of options that can either be installed or not installed and still have an RV-12 qualify as E-LSA. That is why they are classified as options so that someone can choose whether to install them or not. A couple of others were wheel pants and external waiting for night flight.
As for the original question. The ease of installation of an auto pilot as a retrofit would depend on the vintage of the airplane. If it is equipped with dying on Skyview avionics it would be very simple to add the auto pilot. For the most part it just entails purchasing the two servos and installing them.
If the Airplane is equipped with Dynon D-180, it would require just a bit more effort but still not difficult.
Opinions, information and comments are my own unless stated otherwise. They do not necessarily represent the direction/opinions of my employer.
Van's Aircraft Engineering Prototype Shop Manager
RV-6A (aka "Junkyard Special ")