When you upgrade your legacy Rhino to a current version, the new version may not be support some of your plugins, add-ons, scripts, output devices like laser cutters and more.
For example, users with older laser cutters that could print to the device directly out of Rhino 5 installed on Windows 7, found that the same Rhino 5 installed Windows 10 could not do this.
For Rhino 7 or later, you will need a new computer that runs Windows 10 or later for optimum performance.
While the older Rhino 4 or 5 software will not run well on your new computer, your newer Rhino 7 or later software will also not run well on your older hardware. This is why you need to keep both computers.
If you require access to your Rhino 6 after you have upgraded, you will need to licence your Rhino 6 with the CloudZoo method. You will then be able to continue using Rhino 6, even after you have upgrade to a Rhino 7 or later.
Note: You will be allowed to run both on the same computer, However, you can not run Rhino 6 on one computer, while running Rhino 7 on another.