Depends on whether you want to use your W7 elsewhere, and whether your W7 is OEM or Retail. If it's OEM and came with the old PC ready installed, you will probably have difficulty using it on another machine as it will be 'locked' to the original Mobo. Either way you will need to de-activate it to use it elsewhere. That being the case, the new owner will have to buy the full retail version of W8 to install. If, however, you are merely removing the drive for security, and don't intend using W7 elsewhere, the new owner could use the upgrade version and clean install on a new drive as the mobo will still be linked to the W7 install, and will activate an upgrade of W8 using the Clean Install Method.
Meanwhile. if you do intend to install W7 elsewhere, you may be able to reactivate over the phone to M'soft if it's OEM, depends on the operator...
Then there's the Hardware implications as suggested above...