gloopy

I wouldn't outright quit just to go to an interview. Then you'd have to explain your sudden departure and you probably would look unreliable, as well as a bit arrogant. But if offered a class date with a much better airline, the only consideration would be to fall on your sword and give 2 weeks notice (if an earlier class date was offered, which does happen but usually not anyway) or to just bail instantly. If you've already been hired the hiring airline would obviously know that and not care. The only "risk" in that case would be if you got furloughed and were trying to get hired at another regional they might be a bit gun shy about you but that is a very, very minor, theoretical risk in the first place.

No one except the most insecure regional will hold it against anyone that they only spent 6 months at a regional when the reason they left was for a major/legacy. Watch out for training contracts though.