The NG/Liability may mean that if the unit explodes after you convert it to natuarl gas, they don't want to hear about it.

However, I suspect, depending on your state consumer laws, that if any mishap or injury that occurs that can be traced to a manufacturing or design defect could be assigned to Virco as a liability, regardless of what the paperwork that accompanies the kit says. I'm not a lawyer so don't take this as legal advice.

Virco also would have no control over faulty natural gas building lines or extensions, so maybe this is what they are most concerned about, as well as improper kit installation.

I think there is more danger in going from a natural gas setup to a propane setup, however, as propane packs 2.5 times the BTU's per volume as natural gas, and it's at a lot higher pressure, so if you hood up propane to a unit set up for natural gas, you can wind up melting it down or worse.