The change to 4 defenders can happen, in theory, to a 433, a 4231 or to a 41212 (diamond).
I bet Allegri will first move it to the 41212, because it's more gradual - replacing a CB for a CM, instead of a CB for a forward. Allegri has always been very cautious about this, so it's the logic decision.
Therefore I think the move to 4 defenders will rely on the signing of another great/high profile midfielder to play, who can't be out of the team. That would be Witsel, at least on their eyes.
When Allegri had Pirlo, Marchisio, Pogba and Vidal, he even played Vidal as an AM to fit all four, and changed to a back 4 with no problems.
So it depends on Witsel arriving in january, and instantly performing. That's when Allegri will move Pjanic to the AM position and start playing some games on this formation, starting a slow transition.
352 should be, however, the main scheme till 2018, when the BBC era really will come to an end, with the retirement of Barzagli, possibly Buffon, and maybe a relegation of importance of Chiellini.
The 433 won't happen in the near future, imo, because I don't think Cuadrado fits with Dybala behind Higuain and Pjaca will take some time to get full trust from Allegri.
So the transition may start from january, but only if Witsel comes and hits the ground running. If not, it will start only in the 2017-18 season, coming to an end in mid 2018.
And despite seeming a very predictable scheme, 352 is amazing with the CBs we have and specially with Sandro + Alves (when he keeps position width) or Cuadrado (who I love as a RWB, perfect player for this if he can contain his urge to attack all the time and solidify defensive positioning).
Also, there is the Dybala question. I think he really can't play as a winger, and obviously can't stay out of the team. He's a pure old style trequartista it seems to me, going from the middle.
A 41212 with Pjanic as an AM could work from the start, or actually take some time to gel, because Pjanic can't take Dybala's space.
We have to play the way it suits better our best and most special player (Dybala), and with our wingers, 352 is perfect. But Sandro is probably our second most important player in this scheme, can't be left out.