These changes have been anticipated for months. Frontier's CEO, Sean Menke, helped Air Canada introduce its tiered pricing structure (go to
www.aircanada.com and look up a fare and you'll see what I mean). Apparently the delay in rolling it out at Frontier has been technical issues with the website and Sabre, Frontier's reservations system.
Examples of what we might see: a base fare but you can "a la carte" select a reserved seat, frequent flier miles credit, trip insurance, baggage, etc...each for a fee added at the time of the booking.