Nate Diaz = lightweight
Conor McGregor = featherweight champ, going up to lightweight
Conor would not have fought someone who actually fights in the welterweight division. He is not big enough yet. It is not a division he fights in, the fight was made at 170 because Nate could not start cutting soon enough. I'm not sure how much simpler I can make this for Sir_Simpleton. Having one fight at 170 because your fellow lightweight fighter can't make the cut because of short notice doesn't make you a welterweight fighter. It doesn't mean you fight in the welterweight division. It means you have one fight, because of special circumstances, at 170 lbs, against someone who is not welterweight either. He does not weigh as much as people in the welterweight division do.
As for your retarded question, an "up and coming fighter" with one fight in the LW division would be a LW fighter, because it's the division he fights it, it's how much he weighs at weigh in nights, and it's the division he's going to be fighting in.