I am in no way suggesting you do this (because it is amoral not to mention illegal), but what I think/thought had been done to me was that someone bought an HD600 and an HD650, then swapped the drivers, then returned the HD650 immediately after, thus getting his money back for the HD650, whilst keeping the HD600 which would now have HD650 drivers in it. Then I would have bought the HD650 he returned with HD600 drivers in it.
In the UK it is now unwise to do this, because the HD600 actually costs more (off Amazon anyway) new than the HD650 costs new.
Anyway you definitely wouldn't buy HD600 and then buy HD650 drivers, because, obviously you are paying both for a headphone and the drivers of another headphone (which will not be cheap). After the swap, what are you going to do with the HD600 drivers?
I just keep switching them.. And how in any way is it illegal?