The most obvious thing to change tactically for Barcelona is to use speedy 1on1 wingers who stay wide. Potentially they have three: Fati, Dembélé and Trincao.

Messi, Griezmann, Pedri and Coutinho have to play centrally which causes that some have to stay on the bench.

It's probably wise to get another player of this type, especially if Fati moves to #9 (which I wouldn't do).

And Barcelona don't really need a complete overhaul. A must are just the following positions...

CB: Eric Garcia

Piqué, Araujo, Lenglet and Garcia is enough.

LB: Replacement for Alba which "covers" the position until Balde is ready.

RB: Dest and Roberto is great.

MF (three positions): De Jong, Busquets, Puig, Pedri should obviously stay.

Pjanic might be able to help out around three years. Roberto can play there, Alena is here as well.

De Jong, Puig and Pedri will be the future. Maybe one experienced player under 30 might help.

Wingers: Dembélé, Fati, Trincao + 1 signing.

#9: Sign a young nine with clear world class potential.

+ Messi.

So all in all:
* Eric Garcia
* Replacement of Alba
* MF to help transition to De Jong-Puig-Pedri.
* #9/Winger: Depay sounds feasable + (depending on Fati) #9.

So all in all I don't think Barcelona need a complete squad overhaul, they "just" need some proper planning by a board who thinks five years ahead.

And there's no guarantee that Puig and Pedri will succeed but they need time. And Barcelona have time to react in one or two seasons as well.