On the Sony camera,s I would put the TC switch on the side panel to SET,then change the UB and then when still in the SET position change the TC.
This didnt seem to work with the HDX900,the UB would go back to all 0,s.But if I put the button down to REC RUN once,after I set the UB,then back to set and then changed the TC it would keep the UB settings.Just one extra flick of the button.
Just been using the HDX900 in below freezing conditions,and works really well,no problems.And I can fit 7 tapes in my pockets!!!