I think Conte will have had an impact on Poch's thinking. 4231 with high pressing became very vogue and now even teams like Bournemouth can play it, negating the advantage.
A good way to counter a high press is to play with 3 at the back, which gives more passing options to players under pressure in the defence and still allows the fluidity of attackers in the forward position.
I think it's a great move from Poch, and shows he is adaptable to changing trends and is willing to learn. The 4231 was always supposed to be a more flexible formation than 442, but with teams like Chelsea figuring out a new way to stand out, 4231 becomes rigid.
Up for this now. Wenger has always been about playing their game well so they will still create chances, but there is a good chance that our change in formation creates enough uncertainty among their players that they struggle.