Having a quick look at ba.com, I think the answer is yes.

In this case, I think that it will not cost you more to make a change over the phone than to cancel online and rebook online. The price I was quoted for NRT-HKG in W and HKG-RGN in Y was 25,000 Avios + £39.10. This means that if you change this on the phone to NRT-HKG in J and HKG-RGN in Y, you will pay the extra 5,000 Avios + £35.00 change fee + any difference in TFC.

If you cancel online, you will have the £35.00 cancellation fee deducted from what you've paid, and have the balance of the cash (£4.10 in that example) plus all the Avios returned to you. You would then need to pay the Avios price and the correct TFC for the new booking. The net result is the same.