Let me expound on my experience here. As a member of the military who, of course, ends up living in other states than his own, I have had first-hand experience (almost confrontation with one ****** cop!) that you can not be legally required to change your vehicle to conform with standards outside you rhome state under the following circumstances:
1) You MUST have the vehicle registered in your "home of record," or legal residence, state.
2) Your driver's license MUST also be from the same state.
3) You must not be residing or working (on a permanent basis kinda thing) in a state other than your legal residence unless you fall under the Service Member's exception (because the state you're working in becomes your legal residence if you weren't exempt).
4) The vehicle MUST be fully legal by the requirements of your home state AND the federal DOT.
I have had two situations relating to this. One: When I got stationed in CA, we were required to smog inspect the vehicle if it was older than three years old, the vehicle only had to pass 49 state emissions if it was not a CA car (I think it was a way for the state to make some money off us). The other situation was an argument with a CHP officer who threatened me with a ticket for having illegally modified exhaust (which was fully legal in my home state of FL, where the bike was plated and registered). When I basically pushed him into writing a ticket if he thought I was right and guaranteeing I would see him in court, he backed down (at the point of our "conversation" I knew what the laws were.