You shouldn't have to. Once you buy the game, you should be able to download any updates that are pushed here.
Note though, that there've not been any updates lately. The current version was released a few months ago, and apart from maybe a small bugfix, I don't expect many new updates to it.
Chapter 8 will be a new, separate download when it comes, because this chapters 1-7 together have grown too big in size, and I can't really add any more to that download. But chapter 8's not ready for release yet, and I'll put up more info about it all when it gets close.