How unwise is it to change fields/advisors late in my PhD?
Your profile does not say, where you are from. I suppose there are still areas on the globe, where academic titles matter. However, this is certainly not the case where I live. So, in order to provide useful answers, we need to obtain detailed informations.

I still don't understand your motivation! 


If you don't like working in field A, why do you consider taking a job in this field? 
And if you do it to pay the bills, start doing it now. If you're doing a great job in industry, nobody will ever let you go. However, if you underperform, no phd will safe you. 


So I reckon the question you should ask yourself is, why do you believe you need a phd? What do you believe does a phd proof?