Yh I also think this is one of those football myths about not being able to change a large amount of players in one sitting. In reality no one can afford to do it but there's no reason you couldn't - teams change tactics radically when new managers come in and generally you see 3/4 new players a year so why not most of the team? As long as you get a full preseason I doubt it would really affect anything unless you were going to a team that played in a really specific way and had done for many years.
Pep bought Ederson, Walker, Silva, Danilo, Mendy in one window, plus Laporte in the winter. That's 5 starters (when Mendy's fit) & Jesus, Sane, Stones, Bravo, Zinchencko, Gundogan, Nolito in the window before. That's 14 players (not including some of the lesser signings) and if they didn't perform he just bought a replacement. If you have the finances you can overhaul an entire squad, in my opinion, much quicker than people think.