Welcome to club soccer. I'll give you my jaded take on it:
First of all "Flight 1" just refers to a division in a particular league (SCDSL). Flight 1 team rosters (just like Flight 2, 3, CSL Premier, Gold, etc.) change every year as unhappy players/parents/coaches hop from club to club, or as inept Directors of Coaching shuffle coaches around or lose good coaches. Natural turnover happens every year, so every year, the top teams will have changing rosters. The quality or level of a team doesn't seem to have much effect on the roster staying put (unless everyone is happy and the coach is happy, which is pretty rare IMO). I think what is at the heart of your question is the A team vs B team rosters at a given club.
It's a pretty rare club that moves players from B team to A team. I was told flat out by a DoC at one club that if a kid is on the B team (at u12) then that's where the kid belongs. He only wanted the elite athletes with elite size and speed on the top team. After seeing dozens of "try-outs" where the coaches pay zero attention to what is happening on the field except when the kids run sprints, I attest that most of the time, the coach knows who is going to be on his A team within 20 seconds of watching a kid run. Their minds are usually made up based on size at the younger ages, moving to more of a focus on speed at the older ages. The hook that your kid will be rostered on the B team but will always be evaluated with a possibility of moving up to the "Flight 1" or "EGSL" or "ECNL" or WHATEVER, is most often just that. A hook. Clubs make all their bank off of B and C team parents with college scholly dreams, while loading their A team rosters with the local genetic lottery winners, and teasing the B team parents that they will be evaluated every year. Most of that is pure BS. However, there are some clubs that actually do it. Now things get really interesting in the older ages as many of the beasts at younger ages top out and the late bloomers catch up. But none of this has to do with "evaluations" and "try outs." Anyway, my advice to you is not to give two s#$%s about tryouts, flight 1, 2, ECNL, or whatever right now. Make sure your kid is having fun every time she practices and plays a game, has a coach she likes and who treats the players well, enjoy the time in the car on the way to games, and get plenty of ice cream after both wins and losses. If she trains hard, improves her skills, and has above average athletic ability, your daughter will eventually find the right level of competition for her. Generally speaking with regards to level of play, if your kid is the best one on her team by a long shot, she should find a team where she isn't and work to become the best again. Rinse and repeat. Good luck!