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
construct.net/make-games/manuals/addon-sdk/reference/base-classes/iinstancebase
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?