It is pretty much unknown at this point, development hasn't really started on 2.8 version
They are not even finished with the design phase yet, so we don't even know how things are going to be, let alone if they will be compatible or if there will be any sort of "automatic conversion" compatibility layer.
Blender always strives to be as much backwards compatible as possible, though so your files are always guarantied to open, even if parts of it will be outdated or useless.
If any of the proposed changes are actually developed I am guessing cycles materials will mostly be compatible with the current version, mesh modeling and modifiers will probably be mostly compatible too, or at least modifiers will be converted to the hypothetical new "node system".
I am guessing any Blender internal or game engine materials, will be incompatible and need refactoring, probably the same with any game engine logic bricks.
This is at this point pretty much my opinion, based on a more-or-less-educated guess. I read development mailing lists discussions and commits daily, but I am no developer nor do I have any saying in the mater.
Anyway Blender 2.8 is at this point very blurry, undefined, and far away in time, it will probably be another year or more before we even see any working nightlies or alpha builds. If you want to do something you probably better use whatever available tools you have right now and worry about conversion later if you still need it then. In all likelihood you will be done with your project anyway by the time Blender 2.8 is out.