Using either SD or HD assets in the respective opposite environment would require you to import them as a custom asset as far as I know. I have seen custom assets designed for classic graphics work in HD, but have not yet seen it work the other way around.
However, it is in the realm of possibility that you can import an HD asset as long as it is already compatible or made to be compatible (for example, HD uses a different lightning system).
edit: apparently you can force classic models in reforged by appending this to the model path:
war3.w3mod:buildings\orc\TaurenTotem\TaurenTotem