Any ICAO licence with something equivalent to SEP should do. I've come across people with FAA PPL(SEP) licences flying microlights, so far as I could tell legally.
But in my opinion differences training should be mandatory for everybody. Either when going to microlights, or when switching control systems between 3-axis, flexwing, and/or PPC.
In the meantime, any pilot who is not an idiot should get differences training, whether legally required or not.
G