Spectre600
Using body lean styles gives you extra cornering clearance, that is to say, you can turn harder on any bike you use it on. Maximum lean for a bike is set, there is no way to improve it with your riding style. But by leaning farther off the bike, you can turn harder for any given lean angle.
Using nueutral styles gives you less cornering clearance, that is to say, your bike still leans the same amount, but . . .
With perhaps the exception of motard, I believe the three preset positions are (in simulation mode) merely collections of parameter settings you can set in your own custom settings. The two key parameters seem to be where you are sitting fore-and-aft, and how far sideways you move when leaning. I should peruse the appropriate sub-forum
(Wot? No riding-style sub-forum???) to get more input to my own observations, however. I still claim the styles are mis-described. It's really more like "don't lean body with bike" ("lean bike"), "lean body with the bike" (neutral), "lean body more than the bike" ("lean body").
I really think it would be better if the riding style was per-bike, or perhaps loadable from a "settings file". It is interesting that it applies to your license tests, however.
And you should be able to change it in the pre-race (retry) menu, and, really, really, mid-course. (Banked turns at High Speed Ring probably want a different style than the tight esses). But you can't. Maybe when they produce the non-demo version.
Spectre600
I found your question(?) a bit confusing.. You can change your riding style to 4 preset ones in the pre-race menu.
Grr . . . Must be some menu I'm not seeing! I've been changing it in the garage, and leaving it. (Hmm. Have I actually looked under "settings"? Duh.)
That's not there for license tests, though, is it?
Are you sure it's there for Challenge Mode, where I'll likely spend most of my time?