Online date changes sometimes work, sometimes not. An error message is normal if you are no longer in self serve mode with the ticket, but also a long list of other factors. I think you would be best off talking to an agent. There may be specifics about your booking which help or hinder and with 14,000 fare rules on BA it would be easy for me to give you the wrong advice. Clearly if the agent just offloads you for now that would be a first step, but unless you reticket then it will expire at the relevant 1 year point. To re-ticket you need a new set of dates.