When importing C2 projects, C3 guesses the C3 ID by converting to lowercase and adding dashes, e.g. "My Property" -> "my-property". You used an underscore though ("my_property"), if you use a dash it should import correctly. If it doesn't automatically get the right name you can use the LoadC2Property method. But I just realised none of this is documented, so I should update the docs
Now LoadC2Property is documented here
But it's still unclear how to use this method. There's "Script files" property in the c2 version of my plugin. And there's a corresponding "Scriptfiles" property ID in the c3 version. How do I override this using LoadC2Property method?