This issue is a special edition for the 40 years of Iznogoud and "deals with" the story of Mr. Jean Bon of Bagion who can modify the present based on interventions he makes in the genealogical trees of others, that is, by traveling to the past. So he goes to the past, to influence the genealogical tree of Iznogoud - his son's favorite hero - and help him finally become caliph in his place.
The story has the familiar cunning tricks of the likable vizier, intrigues come and go and of course, as you suspect very well, Iznogoud does not become caliph in his place. No way! After all, if the story repeats itself forward, why shouldn't it repeat itself backward...?