I was looking to change to an earlier flight on Monday from LHR-EDI and ba.com were offering me (2 hours from departure) me a reward seat for £13.50 + 4500 Avios. I could have done that, OLCI and printed my BP from the Lounge and just no showed for my original flight. The other option was a £60 change fee at the CS desk but I declined both.

I also changed at the last minute an award seat from SYD to EDI in F in February as there was availability. So it would appear you can, assuming there is availability do it and which often crops up albeit not on all routes and it is a lottery!

Pete