Rhino 7 was developed to not break Rhino 6 plugins. If run into an issue please reach out via the dev forum category.
Trials are only for Rhino 7, when upgrading the previous license is still valid to use. You can also request a legacy key here if you have a Rhino 7 license. Rhino - Access Rhino 6
I notice that you are asking about the other way around. No, it’s not guaranteed that plug-ins that you developed based on Rhino 7 will work for Rhino 6. I’d have to ask a developer but I’d say that it’s pretty much guaranteed that it will not work.
Please contact sales@mcneel.com and explain your situation.
-wim