Dick didn't change under Kory's influence, though. I mean, he did change over the course of their relationship, but not in any way that wasn't consistent with simply growing up. Him rooming with her here, that's the natural next step in the relationship and perfectly natural and inevitable for a social, family-oriented guy like Dick. It's something he would have done at that point if he were dating Babs, or Roy, or Clancy, or whoever. Turning from a slightly reserved, eager adolescent to an open, confident and committed young man isn't at all the same kind of change as completely changing the principles you live by or going from a natural polyamorous inclination (or alien approximation thereof) to restricting yourself to just your boyfriend.