Hi Todd,
I don’t think this should be very difficult. I did a similar modification and what I ended up doing was printing out the building plan and the cutting list and just marking the changes with a red pen. For the most part, you will be building C exactly as they tell you but just some parts are “mirrored”. Read the instructions multiple times and make sure your lengths make sense before you cut them or you might waste some wood. Also, be careful with the parts that are shared between A and B or B and C because I personally forgot about these parts and have to cut wood multiple times. Once I build the frame, I used the instructions as a guide but I had to measure in order to make my cuts. I recommend you do the same and not cut exactly what they tell you in the instructions but cut what you actually measure.