5750 is the lowest i'm allowed to set on a 2k AP using 40Mhz channels. It's also the lowest that CPE's will scan for.
I should of clarified that it's not as if5740 used to be an option on 3k equipment and now its not - but that if you're swapping force 200 CPE with a force 300CPE and the AP is on 5740 at 20Mhz then you won't get it to reconnect.
This is FCC.