Scenes saved in a new version probably won't load in a previous version - going backward is never guaranteed, we can only do "scenes created in an earlier version will load in a newer version." Even if no new features were used, there can be changes to various ways of storing or handling data or memory etc. in the new version that can make the old version have problems - for example, 8 has a whole new way of doing tone mapping. And there is a new parameter in the Physical Material (whether you use it or not).
If the scene does not load, you might (and only might) be able to use Merge and import the objects from the scene into a new scene created in 7. It is always the case that you should never create a scene in a newer version if you might ever want to go back to an earlier version (this would apply to released versions too, so never create something in 7 if you might want to load it in 6, etc.)
