As far as I'm aware, the way the licence works is that basically if you install 10, you cannot then take the key for 7 and use it elsewhere as the licence for the copy of 7 is effectively now the licence for the copy of 10. However, this does not void the licence, you simply cannot use it at the same time as the copy of 10, you can still reinstall 7 provided that you then remove the copy of 10.