I have to say that it is extremely inconvenient for someone who owns two legal licensed versions of Rhino both at home and at work, that files made in 5 versus 6 are not compatible - I had been working on a file in Rhino 6 at home, and now at work (because we have an older version, Rhino 5) I can’t open it. I am exasperated by this and how it mirrors a major problem I have with Adobe as well. Surely, there can be some option to convert (at risk of the user) instead of making the file virtually useless for me today. Please consider how new versions affect the user experience from a file format POV.
Since the V6 file format did not exist when V5 was released, it’s not possible for V5 to read the file.
As you understand, the solutions are to save to a V5 format from V6 when you know a file is going to be edited in V5, or update both Rhino seats to V6.
As a temporary work around, you can install V6 on the home computer using a 90-day evaluation license. That will give you 90-days to decide what to do.