let see now...
i am going to assume you are using the term 'parts changer' in the context that the tech does not have any 'real skills' in troubleshooting and he is just throwing parts on a truck to fix it?

well in a case such as this i think the prudent thing to do is to try and advise 'in a subtle way' to the tech if you see him misdiagnosing a problem on what you think it might be and try to sway his diagnosis in the right direction. Of course in the end he will make up his own mind what he's going to do in the line of the repair. But as far as 'your role' in whether or not you should do anything about it?
If you are an outside contractor in 'thier shop' then there's not much you can do about it. Its thier management's responsibility to monitor and make evaluations on thier techs and decided if the tech is doing his job to the best benefit of the company. One thing I've learned over the years is 'nobody likes a tattle tale' and when dealing with something like this? let them discover and decide what to do about this 'parts changer' tech.
Your intentions may sound noble enough for trying to help out the company by identifying a flaw in thier tech force but being the outsider your information may not be taken so well by any and/or all parties involved in that company's maintenance force.
If it IS that important to you about the situation and feel that strongly about notifying them there is a problem i suppose there are ways to get that information to the right people so 'they can' make that decision among themselves but in the end its going to be thier responsibility to deal with it... not yours. I'm sure eventually the company will see his inexperience and hopefully offer him some training options (if they are a fairly managed company that cares for thier employees well being of course)

As was previously stated, these types of situations can backfire on you as well as work in your favor, not that you'd have any kind of hidden agenda mind you. I'm sure you don't.... right?

so from my point of view i think I might just make an indirect suggestion or two and then let them sort it out themselves. You do your job and let the others do theirs and don't meddle in their affairs that don't really directly concern you and your role there too much.

I CAN fix forklifts, i just CANT fix people, trust me i tried for years, it just doesn't work and requires way too much energy i could be using for other things i CAN do something about ;o)