Does somebody know what has changed?
Good question. It's pretty much just corrections and clarifications.
So most of what has changed is matched in either the errata or the FAQ. Everything else was just cosmetic or slight wording tweaks.
The only "new" thing is the rule that players can't look through their discard piles. This has been a rule for a long time, but apparently the cultists cut it out of the book itself and no one noticed until now.
If you've already learned the game then I'd just go through the errata and faq and adjust your brain with those. Then use the new PDF for reference when you need to look something up.
If you haven't learned the game yet, then just start with the newest version of the rulebook.