It depends on if it's a bolt on, neck through or set neck guitar. THe latter two can't be replaced, and neck though guitars have the neck manufactured out of the same piece of wood as the body. So unless you've got a vaccant neck cavity, you can't replace a set neck. Bolt ons I think you can unscrew but I wouldn't!
Current Rig:
Gibson Firebird Studio
Limited Edition Schecter 35th Anniversary C-1
Schecter Jeff Loomis Signature 7 FR
Ashdown Fallen Angel
Custom 7 Firebird from Ignition Custom Guitars (check them out)
ESP Phoenix