Hi Bernardino--the AD could not offer a swap, and that's the norm: I think you're unlikely to find an AD who would be able to do it. However, there are two OBs in London, and I phoned both with the question; the first said 'impossible', the other said 'no problem, we'll do that for you if you buy the watch here.' (I can't remember which was which.)
I stuck with the AD (Beards of Cheltenham), who offered a considerably better deal (bracelet swap or not) and were just lovely to deal with. My qualms about the sapphire bracelet are now completely gone, so I'll keep it. However, should you wish to swap yours, I think you'll have no problem at all selling it. Whereas there are always brushed/polished bracelets for sale on Ebay, my impression is that the fully brushed version comes up for sale very rarely, and gets snapped up right away.