The CTRIO engineer will have to correct me if I'm wrong, but I believe the 50KHz option was removed because we couldn't guarantee a flawless (i.e. smooth) output without stalling if the CTRIO's inputs happen to be configured to do scan-intensive things like Edge Timing and Rate Scaling. Also, CTRIO Workbench configuration became very complex if this feature was allowed... something that I don't think seemed favorable at the time.
As far as downgrading to an earlier version, I don't think a CTRIO firmware version ever made it out the door that could do this. I'm pretty sure that it was prematurely stated in the manual and then quickly removed.