The root of it is what Sendric said, yeah. Anything reprinted with the same name is treated as errata, and we had to implement them as such, as well as a few base name changes such as positive/negative becoming vitality/void, and some language names that got changed. There were some technical influences too, namely that since all renamed content had to be treated as errata for purposes of PFS, it was going to be extremely difficult to maintain distinct sets of errata'd and non-errata'd content based on remaster/PFS/non-remaster (not to mention the aspect of having things that the files previously treated as unique now have two versions). Having gone through all the content though, they're basically all cosmetic changes or some minor balance tweaks, largely nothing fundamentally changed.
Main change that affects all characters is probably the removal of alignment. So much stuff got updated/errata'd to reflect the removal of alignment, it wound up being much easier to remove it to reflect Paizo's change to the game system, and to improve our communication of edicts and anathema. Champion is the most heavily influenced by this, as they got a large compatibility document of errata for the class, though again, it looks like a lot but kind of amounts to they can now hurt some extra things with spirit damage that they couldn't hurt before with good/evil damage.
