There is some confusion in the rule language since the old transponder rules specify "engine-driven" and the ADSB ones do not. Here's a recent letter from FAA counsel's office confirming that if an aircraft was transponder-exempt under the old rule, it remains ADSB-exempt under the new one.
Source: https://www.faa.gov/about/office_or.../agc/pol_adjudication/agc200/Interpretations/
Source: https://www.faa.gov/about/office_or.../agc/pol_adjudication/agc200/Interpretations/