You can go Villain - Rogue - Vigilante - Hero. Null the Gull will change your alignment just for the asking, as many times as you like. ("What can you do?", then Change alignment.) You can also do the Tips missions method starting at 20, which in Live was the only way.
Villain and Rogue have redside contacts, Hero and Vigilante have blueside contacts. Rogue and Vigilante are "in-between" alignments, and can visit both sides and participate in both sides' content. As soon as you flip sides, you'll gain contacts for that side so you're not out in the cold, but your old contacts will go inactive.
My main runs Vigilante most of the time, because she's nominally heroic but takes a more cosmic and pragmatic view of things and has no time for idealism. She can't initiate redside missions or TFs (because she's still blueside), but she can participate in them if someone starts.