I have an undergrad and PhD in a STEM field (engineering), and a few publications in some decent journals. since passing my PhD, i've been working in a management position in a tech company for the last few years, but needing some mental stimulation / diversification, started doing some papers in economics and philosophy after hours. I've done pretty well and i'd like to continue diversifying. After many years of self guided research, being back in the teaching environment as a student has actually been very cathartic and enjoyable and it turns out social sciences / humanities seem to come more naturally to me than i had thought previously.
My question relates to whether it would be wiser to start in a new field 'from the bottom up' (ie undergrad) or try to get into things at a higher level. I dont want to be bored but i also enjoy the rigour of starting at undergrad level. I'd like to get some opinions on this since most of the academics i know tend to stay in their research fields. I should note i never intended to do a PhD but was promoted from a masters after my field year. I also seem to believe that education is not only about specialisation as is more common in the modern era, but more in tune with academics who covered many fields during their careers for instance Liebniz (law, mathematics etc)
Are their any PhD's out there who have diversified effectively into a completely new field? What is your advice? Did you end up 'starting again' ?