Lemme get this straight, you need to start a new world in order for the update (adventure update 1.8) to take place?
Once it's my turn on the shared map I'll be greatly expanding the size of our world, because after 1.8 happens any new chunks generated will have errors, if I understood Jeb correctly. But we'll have to stay inside this new area forever, until we willingly decide to change maps of course. But I'll be making the map pretty damn big (100mb gives us a lot of space), so it'll be quite a while until we really get tired of it.
But if the 1.8 update also includes super amazing changes that makes 1.7 and earlier maps look like crap, then we can also change if people want to.