I'm using FA 2016 and went through the biochem section just to highlight new stuff. This took me about 2 hours (60 pages or so). I still have to write them into my old FA, as well as read up on anything thats new to me. My guess is the entire process will take around 4-5 days.
As for the changes being important or not, I'd guess its the former. Information doesn't get added randomly. I also know several people who've mentioned how they missed a couple of easy questions because they'd just been added to the newest FA, which they decided to skip.
The problem isn't the information itself (its easy, since at this point, having read FA 2016, assimilating new info isnt difficult), but the process of filtering through the 2018 version to find everything. I simultaneously want to and not want to do it.