I've been 100% on Precheck since getting GE in 2013, although when I started I had to have several reservations adjusted that were ticketed before I got Pre. US Airways (at the time) was able to add my KTN to all existing reservations when asked.

What a lot of people don't realize is it doesn't matter if you have GE/NEXUS/Pre current and on your profile when you check-in/fly. It has to be current when you fly, but the KTN is submitted at the time of ticketing, so if you book a flight before you add your KTN to the profile you still won't get Pre even if you got GE/NEXUS/Pre in the interim.