To answer this question, you need to be clear on what “postcolonial” means. In essence, it means that colonialism is over. So, in the context of this story, Algeria, where “The Guest” takes place, is no longer under French control; it is a free and independent nation.
With this in mind, “The Guest” would look very different, especially the ending of the story. Remember that Daru has been tasked by a French gendarme, Balducci, with escorting an Arab prisoner to police headquarters in Tanguit. Although he sets the prisoner free, giving him the choice over whether or not to hand himself in, Daru returns to the school to find a message from the Arab rebels. The Arabs promise that they will make Daru pay for handing their brother over to the police.
This dramatic climax would be very different in postcolonial Algeria, because if there are no French authorities, then there would not be any Arab rebels, either. Even the character of Balducci could be affected by postcolonialism. Remember that he is a native of Corsica and would perhaps no longer have a reason to be in Algeria once it became an independent nation.
Thus, in a reimagining, the ending of the story would have to change significantly to reflect the fact that Algeria no longer needs a nationalist presence to agitate for the end of colonial rule.