They started changing when they were 15, and getting into this rebellious stage with the whole "Rebel Missions" (along with Zack Hecker, who by the way I strongly stay by my statement believing he would have been the 3rd shooter if he didn't change, nor if he didn't start dating Devon).
Eric and Dylan met Chris Morris in 10th grade. Chris Morris got Eric the job at BlackJack Pizza after Eric's birthday...sometime from May '97 - July '97. Eric got Dylan working there at BJ Pizza from September '97 - December '97 (Dylan quit because he brought a pipe bomb to work, and was yelled at by his boss. Dylan probably said "Fuck this" and quit). He only lasted 3 months at Black Jack Pizza, and didn't start working there again till December '98
Anyway, Chris was one of the founders of the TCM. He was one of the first guys wearing the Trenchcoats. Guess what year? Eric and Dylan's 10th grade year. '96-'97. Eric Dutro was the first guy who wore this trench coat in Halloween, and it became a new thing for them. Eric and Dylan respected TCM, and probably always wanted to be in the group, but really never was in their group, and IMO..kind of wannabe TCM guys. Chris was said to be very violent. He often cussed out teachers, got in trouble a lot, pulled out a knife on a fellow TCM member (Eric Dutro), and had one hell of a temper. The more Eric and Dylan hung out with Chris (especially Eric. Eric was closer to Chris than Dylan) the more that Chris had an affect on him.
Now do I think Eric was Mr goody two shoes before Chris Morris? No. I think Eric always kind of had that Rebellious nature in him, but I do believe Chris did influence Eric into changing his style. I mean even look at the sunglasses Dylan and Chris both wear. While Dylan's does look more expensive (it is, he paid $300 for it) it is similar to Chris Morris sunglasses, if not exactly the same besides the framing.
Just my two cents