For the most part, iClone 6 will still work with 3DXchange 5. However, there is no way to export iClone 6 files to formats such as FBX/BVH/OBJ due to the data format changes with new iClone 6 content. Therefore, iClone 6 files only can be exported to external formats in 3DXchange 6.
Version Comparison Chart: -
http://www.reallusion.com/iClone/3DXchange.html