It's my understanding that you can change anything, including the first segment, at no cost prior to beginning travel (for NA originators, 7 days before that). It's been said here that if you make significant changes after 15 April you will still fall under the pre-15 April rules, but I'd worry about that until it was a done deal, and try to have the strawman itinerary as close to something you'd be willing to fly as possible. At worst, you can always refund your ticket and not be out much.