My take on it for what it's worth is that for a large part of the season we have been playing with 4 at the back and one of the full backs pushing into midfield to add to the numbers. We did this with Walker, Cancelo and Lewis. So we have basically been playing with a back 3 for a long time.
The big difference is that it appears Pep has gone thought rather than pushing a defender into midfield just start with an additional midfielder and make us tougher to play through.
I have no problem with it, truth be told I am quite interested to see how it pans put.