Sorry to hear OP's situation. Definitely you need to call OnBusiness during their opening hours (whatever those are as stated on the BA website).
IME OnBusiness were helpful when I had to change a booking date before departure (though also like OP after initially wasting time with standard BA CS hotline). My booking was a sightly complicated one in that it was a cash WTP booking upgraded to CW using OB points. The OB agent knew what she was doing and changed my flight without any problems.
In fact got more than I bargained for as the flight rebooked into R class, a paid business fare basis, so I got more Avios and TP than I would have done otherwise
