PAXboy - you're probably more knowledgable than me on this. Given that LTN and STN are not that far apart, I don't really understand why an airline moving a route that used to go out of STN to instead going out of LTN would cause significant risk / costs.
Yes, you have to tell people already booked about the change of airport, but I imagine most people booking with U2 or FR would be indifferent between the two when it comes to booking.