That shouldn't be our worry at all. If Allegri thinks it's doable, he should do it.
Cancelo will cost 40m. It's not cheap, you could find a winger for that sum as well. SMS is expensive as $#@!, so again: I'll fancy such a creative player as CM and we can easily find a winger to fill the gap (in fact it'd be one that's much cheaper than SMS).
Players change position more frequently these days and it's out of necessity, because it's not easy to buy in this market. At the end of the day we're lucky to have both options available (on one hand buying and on the other moving players to different positions).
Bernardeschi would be a beast B2B if he ran a bit more and could actually defend a bit better. The guy is superb with the ball - control, passing, vision, dribbling when it comes to your typical B2Bs. He's not lacking anything physically so it could be done
Cuadrado is more difficult because IMO he's a little bit too lightweight for a RB, and goes down into sliding tackles too easily. It's still doable though, Cancelo is not actually a classic RB either - afaik he was used as RM before.