I think Alonso will change the system, because I don't see him finally deciding, at the Club World Cup he played with three forwards, Vini, Garcia and Bellingham, then Mbappe was injured and Rodrigo spent almost the entire tournament on the bench. Now Bellingham will be absent for some time after surgery, but he will still have enough attacking players to play even with three forwards. If with two then it is even easier, the most important thing now is the midfield.
He surely is going to change the system because that is the only way he can Introduce his own system to the team for then to achieve the kind of results he wants of them to achieve by the season. At this time he is going to be using more of rotational tactics to enable him get his players at their peak at almost all time throughout the season pending when the likes of Bellingham will be back to joining the team and the new players who just joined the team and are trying to fit in with the orientation of xabi.