In the end, it isn't going to matter, as there will be plenty of opportunity for everyone, regardless of seat. Get the app ready, hone up on the knowledge skills, and prep. If you wish to make the jump, I'd say you won't have an issue obtaining an interview. However, it doesn't guarantee the job.
I've heard about "moldability" and "trainability" when it comes to high timed pilots. However, I thnk things are a bit different now, as the age 65 change has stapled 5 years of complete stagnation on to a large percentage of DCI pilots. The result is that the majority of us now have 4k or more hours that we'd rather not have at the regional level.
I've known many First Officers that have moved on, but they have all had a former background at DAL in various other departments there. I think this is their "in", so to speak. However, I'm not sure it is the "norm" or DAL's "preference" to hire First Officers. All I know is that I know way more former Captains that have moved on, than I know of First Officers. In the end, I believe it is heavily dependant on your networking skills, regardless of seat.