SWw always had an ambiguous relationship with reality, think only of most protagonists' shooting abilities. But every genre, every single movie must have some kind of 'inner logic', call it a set of rules to play if you wish. Without any logic, without any rule a narrative art form is unimaginable. Even breaking the rules is confessing that there are such things as rules.
As far as SWs are concerned, it will be clear that another inner logic/set of rules is applicable to a) The DOLLAR trilogy b) The SARTANA/SABATA movies and c) The TRINITY movies
What surprised me about Johnny Yuma, was that it wasn't consistent concerning its own inner logic: the completely silly barroom brawl would've been more appropriate in a c) movie while the bulk of the action definitely was an a) movie (with some influences of the b) type). Johnny Yuma is not the only SW with this kind of inconsistencies, but that doesn't make them less distracting. At least to me.
This changing guns/gunbelts and shooting with both hands by Damon is of course something else. I don't think it was a mistake (as suggested by Cian) so it meant something, and I was wondering what Guerrieri and Di Leo were trying to communicate. I already stipulated the symbolic function of the scene(s) but still felt a bit puzzled. Maybe it also was a reference to Clint Eastwood in FOR A FEW DOLLARS MORE in which his character does everything with the left hand, except shooting. The script of Johnny Yuma has some similarities with Leone's film so a reference is quite possible.
But I'm not sure about it and it keeps puzzling me.