For now, there is no much difference between a v5 and v6-to-come pcb file, but things will get worse probably in a very near future.
The most visible difference today is the the inclusion of a “max_error” property.in the setup section. You can try to remove it (after a backup, obviously) and try to load it in pcbnew v5. If it works, don’t forget to refill zones (b key).
There kind of where massive changes already behind the scene and recently some changes where made to the file format. In particular to how zones are handled making nightly files no longer able to be correctly read by stable builds. Other changes will follow soon. (Yes there could be time invested in keeping a fallback mode but that would take away resources from other stuff. Nightlies are simply not meant for production work.)