if it worth dissing someone else go for it.... no toy is gonna care if he gets dissed by a good writer, and if for some reason, tha toy managed to work on his shit, and get to the point of being okay, i doubt he would care if you went over his old ass weak shit... he should be embarrased of it...
i know i wouldnt mind if some of my early trains got dissed..... i kinda hope they do...
i think as a person interested in good grafffiti on trains, you have an obligation to go over the bad shit... there is way too much of it, if you can find a shitty piece, put a good one in its place, it will help clean up the lines...... something that needs to be done...