Yes I guarantee that you can downgrade from Official OTA to RC29. Like I said I did it yesterday morning with no problem. I have used that same .nbh file since I started rooting RC30. Did you try using the one I posted?
T-Mobile HTC One successor to my T-Mobile Sensation 4g Successor to my TBD HD2 Successor to my EnergyRom TP2
OS: Android Revolution HD 7.4 | Sense 5 | High Quality & Performance | 1.29.401.13
Kernel: HTC Stock for now
and don't forget to thank me if I have assisted you in any way