Mmm. I'm not so sure. My licence says B737-300-900, however my rating insert in the licence states -700. If I want to fly any other vasriant then I need to pass an OPC/LPV on that variant anf thus get a new entry on my ratings insert stating that variant. Before I do that there must be a diff course, even if it's just a few hours g/s followed by a few more in the sim.
But yes, 300/500 to NG would definately require a diff course.
Interested to hear what others say.