Does coop play require buying D2 Resurrected?

Well, I’m sure that even if there isn’t an official way to transfer original local characters over, someone will cook up something similar to the Hero Editor that will at least offer a pathway for you to recreate your local character on D2:R, or at least some sort of conversion tool.

If there is a high enough demand for local character conversion, Blizzard may decide to create a conversion tool, who knows?