No medical needed ONLY if the pilot certification doesn't require it. You can't fly under a PPL (or higher) with just a drivers license because at that point without a current medical rating your pilot certification isn't "current". You have to have the medical rating that coincides with your pilot certification or you are not technically current. That came from our local FSDO here about 3 months ago. Being "current" is the key factor. I was trying to avoid the flight medical and thought I could use the "drivers license" portion of a SPL and then utilize my PPL certification to satisfy the Section 333 requirements. Got a big NEGATIVE on that because the medical aspect is an integral part of "currency".
I have been doing some research on this also. I found this from EAA, and contacted my FSDO who agreed with it. I also contacted a Sport Pilot instructor and he also agreed.
Existing Pilots Becoming Sport Pilots | EAA. I will be getting my Flight Review in a Sport Pilot Aircraft with the proper endorsement in my logbook.