I have done this before on a "Business UK" fare. The ticket desk was able to accommodate my request, but make sure you turn up at the ticket desk well in advance in order for them to process you. In my experience it did take some time for the agent to amend the booking and get me on the flight.
I must say that I have only done this once (change of day) and I am unsure if this was a one off and the agent was bending all sorts of rules to get me on the flight - it may not be the norm!