Big Blue was Champions 4th, not third.
Champions 4th to HSR 5th was mostly more concrete examples and all the FAQ elements being added, while removing some of the Champions-specific setting material.
HSR 5 to 5th REd aka FREd, mostly corrections to the examples.
HSR 5 to 6 was making all the figured atts no longer figured. It was an attempt to expand the fanbase. Most of my friends who play hero refused to even look at 6th, and continue to use 3rd, 4th, or 5REd.
There were a number of things in 5th and 6th that were attempting to address old weak spots in Hero, and they mostly did, though in some cases (6e KA) they may have over-fixed it.