Nothing much The all used to all new is a simple evolution of economics. I started with a gift (Konica FS1 + 50 mm F1.4) - somewhere about 1980 when our firstborn was small. A kit I extended as I came across s/h Hexanon lenses and a second body all of which were very cheap as Konica was a rare fitting where I was and the shop was only too pleased to shift any that came in. Pride of place was a 80-200 F2.8 which was wonderful. The whole lot got nicked (we were burgled). With the insurance claim I bought a second hand 2 body Minolta kit with primes from 28-200 covering everything lost plus winders (the FS-1 was I think one of tbe first cameras with a built in winder and what is now a standard shape grip). I never really got on with the minoltas but they were used until they died. The film advance went on both. I then bought new a Canon 50E with twin lens kit (28-80, 70-300) and most wonderfully autofocus. I still have it. I then bought an ex-demo Bronica 6x6 + 80 mm six months before they went out of business. I added a 50 mm and spare 120 mag s/h. When I "went digital" in 2007 I bought a 5D new and from that time I have bought sparingly, new, when I needed something. If I was into collecting or indulging fancies then it might be different. I don't much see the point of duplication although I do have two 70-200 zooms. I was working abroad for 5 years and got fed up with the lens always being in tbe wrong country when I wanted it. So what was 100% bought used out of financial necessity is now all new bought and well used.