I was also given 2nd to last row middle seats on my last two F9 flights after checking in at T-24. I just asked the gate agent to reassign my seat and they were able to do it for free prior to boarding both times. I also saw a couple ask the agent to be seated together, and their request was granted as well. I've seen other LCCs do this too - charge for seat selection when booking, but allow you to change it for free at checkin and/or at the gate.