If I have to modify most of the proof, I will use the entire proof (with proper attribution, of course.) However, I don't believe I will need to do that. So, how much should I show of the original? Could I do a line-by-line comparison of the changes? If I only change a few variables, could I do something along the lines of, "By changing variables a, b, c and relaxing condition x, it's easy to see that Erdős' proof arrives at the same conclusion without breaking the original."
Some suggestions would be appreciated. Thanks.