The electrodynamic bits .. 'burn in ' I am quite skeptical of; an hour or two, at most.
However - as above - the 'fit' and very especially, the pads finding their natural shape against your head to seat/seal propoerly, is a huge part of it - regardless of 'open' or 'closed' the close-fit does affect the response. Your own comfort when this is established also let one relax and just enjoy the music; that's physio/psycho logical more than a change in sound, but no less real.
I replaced teh pads on HD565Ovations I'd had 20yrs - and they took dys of use to soften/take up my skull contours. I cannot say the sound changed at all in that time (- long in use!) but one day - oh, now comfy! = huge change in resulting gestalt.