I'm in Houston area so similar climate. I have Barred Rocks, Easter Eggers, and a few others so similar breeds as you. Like Bill said, every chicken matures differently. All you can do is wait and see.There is no guarantee
There is no guarantee when they will lay. A lot of things play into it like breed, climate, and just the chicken itself. @WingItRanch's took 25 weeks from my calculations. I don't know what kind they were. So, no guarantee, but you can tell when they are getting close when the comb and wattles get pretty red and they start showing interest in the nesting boxes. Or, they may even start squatting for you. You didn't mention a rooster.
I wouldn't switch to layer until all are laying eggs. Honestly, I wouldn't switch to layer at all. I prefer to keep the protein higher and just provide oyster shells on the side. You can put oyster shell out anytime. They'll take some if they need it or ignore it if they don't.