If you (or the person you are talking about) has the correct licence for aircraft "type" then it is basically a checkout unless "differences" training is required. So for instance if you have an SEP rating and normally fly nosewheel simple SEP's then converting onto another simple nosewheel SEP, it is just a checkout and can be done by another PPL or you can even just jump in and fly it without a checkout (legally I mean). If you are going to a different "type" like TMG then it has to be an instructor.
I'm pretty sure that's correct, but things are changing so often now, it's a struggle to keep up!
SS