I'm not sure there's actually any evidence that they 'change shape' to conform to your arse as they break in whatsoever.
People say they get comfier, but that's largely because you're getting used to it.
Brooks may SAY that they change shape to conform to your arse, and this thread perfectly highlights the motive for this claim!
I've read lots of threads on yacf about people who've bought a second hand brooks and been happy with it, but I've never read a thread from anyone who's bought a second hand one, hated it, but then bought a new one and liked it.
That said, if they are £35 there's not a huge amount of point not buying new.