The "why" of the two changes you mentioned seem obvious to me. Ease of suiting-up.
The Kidney/ Ab plate are now joined and are free from the back and chest plate. Much more freedom to twist about. The diagonal line to the join is a mystery to me vs. a vertical, but there may be some attachment\ mobility gains happening there as well. The black band between the chest and back turn that top piece into a shirt. You can see the shoulder straps are molded into each from the close-ups meaning it's supported from the shoulders now as a unit rather than from strapping. It makes it easier to toss on and off quickly and, I suspect, more of a one-size-fits-all application. Most of the other changes will be due to the high-res nature of modern filming. The OT was in really crappy resolution, even on the big screen. 35mm film when projected to that size was better than the old 480p some of us grew-up with on Tube TVs but was, typically, worse than even 720p HD TVs. (yes, there would be outliers but this is typical.) http://www.motionfx.gr/files/35mm_resolution_english.pdf When you look at Blu-Ray stills it's obvious how cheap the TK armor was. It looks bad even for the 70's but it's what they had because of the budget Lucas had to work with. There's no way it'd fly today with modern audiences on 2-story IMAX DLP screens in 3d. That's just the way of the world.
The helmet shape and armor profile looks close enough for anyone but people who've built one to say, "That's a Stormtrooper." Good enough for me.