They won't talk to you if you are on the opposing faction. If you try to talk with Mr. G as a Resistance member he will tell you to go pound rocks (not literally, but that would be cool if he did). Same thing with Dr. Hetzfeld and all the other "Power" contacts. What you really need to be careful about (I have done it once and it is a pain to fix) is switching factions before completing the (Crusader/Power) arc and not having any other arcs left in the zone. I once switched to Resistance on my second to last arc in Nova. Unfortunately, the only arc I had left in Nova was Reese's and as soon as I switched to Resistance the mission and contact were gone and inaccessible. I had to run through the entire Crusader arc in Imperial with my XP off and stuck at level 10. I betrayed Wardog and switched to Loyalist then went back to Nova and beat up Reese. I was almost halfway through Imperial City by the time I hit 15.