You should be able to change the flight but may have to pay a fee.
As long as you make the changes more than 24 hours before the flight departure you should also get your BA miles back if you end up opting for the economy cabin.
See relevant BA Ts&Cs below.
14.1. Changes to bookings for Award travel on flights operated by British Airways or Airline Partners are permitted only in respect of date, time, class of service and (subject to Clause 14.2) route. Such changes may only be made by the Member or an individual nominated in accordance with Clause 3.13. If more than one Part Cash Part Miles price option is available for a British Airways flight, it is not possible to change the price option chosen by a Member after a booking has been made. All permitted changes are subject to capacity limitations and may not be made after the time 24 hours prior to the scheduled departure of the first flight in the Award travel itinerary. A fee as specified on ba.com may be payable for each permitted change. For the avoidance of doubt, name changes are not permitted.
15. Cancellation of awards
15.1. A fee as specified online at ba.com will be charged if flight bookings for Award travel are cancelled by the Member prior to departure. Provided a cancellation is made more than 24 hours prior to the scheduled departure of the first flight in the Award itinerary the Mileage charged for the Award will be re-credited to the Member's account.
15.2. If flight bookings for Award travel on British Airways or on flights operated by an Airline Partner are cancelled by the Member within 24 hours of departure, Mileage in respect of the Award will not be re-credited to the member. In the event you are prevented from travelling by an Event Beyond Your Control we will refund your Mileage provided you have an unused Award, have informed British Airways promptly, provided evidence of the event if requested to do so and paid the applicable fee specified on ba.com.