This has been gone through before.
I was exactly the same when mine turned up. Had test driven with the old style sliding gate version, and mine turned up with the new style leather gaiter.
Anyway, I spat my dummy at the dealer and they took apart an old style car they had and their new style one and compared the two.
Long story short, it can't be done. Top of the gearbox is different, as are the electronics that go with it.
New style also benefits from gear indicator on the DIS rather tha just 'D'.
Having lived with it for a while, personally I'm very glad that they couldn't change it.