Not necessarily. Some door seals clip or slot into place. Even seals permanently attached to a door can be replaced as in this video-
http://www.espares.co.uk/advice/fridges-and-freezers/a/2/1145/how-to-replace-a-freezer-door-seal.html
If you can get a replacement seal, the seal can be replaced- obvious, isn't it?
Smiling Carcass
July 2010