It's...the same skeleton with a better skin wrap (I can't really think of a great analogy here). It is the same fundaments, the same base, but they changed enough of it that there's a bit of learning to do. It's not like going from 2nd Ed 40K to 3rd, more like 3rd to 5th.

There is no progression to the background as it just rehashes what was covered in the previous core book and Human Sphere book (aside from the addition of Tohaa Sygmaa and the removal of the Exrah, which get some explanation). Although, the next book is believed to be an entirely new book as opposed to Campaign Paradiso N3, so will progress the story, which each book release typically has done.

OH! Analogy! Base model Chevy Tahoe (almost typed Chevy Tohaa ) compared to a loaded Cadillac Escalade. Same chassis, motor, etc, but vastly different vehicles as far as options and driving experience.

TL;DR: If you played N2, N3 won't be a terribly difficult transition, but as with all things Infinity, there will be a learning curve that one should prepare for.