there is a HiTec Rotation Modification Difficulty List, I cant remember where I got it but I'm sure you can google it. Lucky for you I put it into Word, the 311 gets a difficulty rating of 5 and the 425 gets a rating of 1.
Difficulty: 1 is the easiest, 10 is the hardest.
1-3 rating: Some very slight modifications. Trimming of plastic with a knife or dremmel.
4-6 rating: Trimming of plastic with a knife or dremmel. Drilling out of final gear/spline.
7-9 rating: Pulling or tapping out of metal pin from metal spline gear. Possible soldering.
10 rating: Drilling out of spline gear. Trimming inner case stops. Trimming spline gear stops. Possible soldering.
anything up to 6 is pretty easy even for a beginner.