Design Patterns to Handle Versioning

I am building software which references a DLL containing ~10 classes each representing a message. Each message contains fields that are specific to that message type. Every six months or so updates are made to each message type and I am looking to make the software flexible enough to smoothly handle these transitions. I would like to be able to select a version and a message type from a drop down menu in the GUI which would then reference the given version of that message type. I have a copy of the Gang of Four text and am trying to find a suitable solution to this task. Any help would be much appreciated.