How can i force the client to grab the latest version of mission

Having messed arround with a client and server running on two machines - ive made a basic test level but find that if i make changes to the terrain (the rest seems to work fine) the client downloads the mission but then squarks out

"You do not have the correct version of the FPS starter kit or the related art needed to play on this server, please contact the server operator for more information."

this doesnt appear to happen if the changes are just moving a few objects arround (interiors etc) though i imagine it would if i changed a model/replaced with a new version

Is there any way - in a current resource or whatever - to force the client to load the latest version of a mission in its entirety?

I supose what im asking is has anyone got an ingame patch system working and if you could point me in the right direction?

Thanks heaps :)