Like I said, any shop can get the parts through Apple. You just have to: Have a Apple Hardware certified individual and apply for the account.
Drives do indeed get flipped. If you speaking of situations where: User machine fails, pull hard drive out, place in comparable machine. This is most assuredly possible with the retina as well.
User has rMBP, something fails, pull the drive out of the rMBP place in another rMBP. Now this doesn't work across models naturally, but the basic premise is there and generally with Mac OS you don't want to flop OS installations between different models anyhow.
Pulling data off is obviously troublesome, but doable assuming we aren;t talking a drive failure.