Nope, you were not correct
You can run the Insider builds on any machine that is capable of running Windows 10.
You do have to sign-up for the Insider program or the Technical Evaluation (Enterprise edition) and
you do have to agree to the terms.
I think there is a lot of confusion between the Insider program and upgrading a qualifying OS.
If you upgrade an OS, why would you want to run Insider Builds?
To answer my own question, you might want to continuing testing or at a minimum see what's going to be offered in the next Current Build on Win10.
You wouldn't want to over write your newly upgraded system though (some might ????).
The better way would be to dual boot Win10 Retail (Current build, RTM, whatever name works for you) and Win10 Preview as another boot option.
You could also run the Preview in a Hyper-V machine of other VM, but I had some issues when a new Insider build was released - Windows said I couldn't update a VM, so I Clean installed the builds when the ISO was offered. I eventually changed to a dual boot configuration.
But no ... you do not have to upgrade a previous version of Windows to run Insider builds.