Corona 7 warns you (and offers to make a copy) before saving over a scene created in an earlier version of Corona. If it was a new scene created in 7, then of course there is no warning. Compatibility never works in reverse, where something created in a new Corona will work in an old Corona, so never ever create a scene in a newer version if you will have the need to go back to an earlier version at any point (there are often under-the-hood changes too, even if there is not a jump like the Physical Material).
You can start a new scene in Corona 6 and Merge the contents from the Corona 7 scene, and redo the texturing on the imported geometry to use Corona 6 materials. BTW, Corona 7 costs no more than Corona 6, it's all part of the same subscription, you can use any version you like while that is active.
