Paris fashion week starts today with dizzying changes at the top of some of the most famous brands. Saint Laurent, Dior and Lanvin all have new artistic directors who will be putting on their first shows in their new roles.With more than 90 runway shows and dozens more presentations behind closed doors over nine days, the spring-summer ready-to-wear collections promise to be the most scrutinised for some time. Here is our guide to the shows to watch out for:How will Anthony Vaccarello face up to the massive challenge of filling Hedi Slimane's Chelsea boots at Saint Laurent is the question on many fashionistas' lips. Slimane, the man credited with reviving skinny jeans, brought the French label a whole new younger generation of fans, drawn by his grungy rock star chic.Nor is Vaccarello a slouch when it comes to sass. The 36-year-old Belgian has developed a leggy and asymmetrical style since he first came to fame with a collection inspired by the Hungarian-Italian porn actress turned politician la Cicciolina. And he showed he could do rock with the best of them with his designs for the younger Versus Versace label. Like Slimane, he has a taste for black.And in a sign that he is prepared to sink all his creative energy into Saint Laurent, Vaccarello has put his own brand on hold. All will be revealed at the Saint Laurent show on Tuesday evening.The popular French haute couture designer Bouchra Jarrar debuts today at the head of Lanvin, the oldest Paris couture house of them all. The 45-year-old takes over from Alber Elbaz, who was fired in October 2015 after 14 years at the head of the label, in a move that caused dismay at the time.The label's redoubtable Taiwanese owner businesswoman Shaw-Lan Wang has now turned to Jarrar, and her pure, elegant lines are likely to mark Lanvin's show on Wednesday. Jarrar, whose parents came from Morocco, worked her way up through Balenciaga and Christian Lacroix before launching her own label in 2010.But perhaps the week's biggest moment will be Italian designer Maria Grazia Chiuri's debut show on Friday at Dior as the first woman ever to lead the house. The 52-year-old helped revive Valentino with her creative partner Pierpaolo Piccioli before leaving him to run the Roman brand alone this summer.Like the Belgian Raf Simons who she succeeded, hers is a sober, restrained aesthetic with romantic flourishes, very much in contrast to the flamboyant theatrics of John Galliano's time at the helm. Simons has since moved on to the more mass market Calvin Klein.French designer Christine Phung is the other new arrival of the week. The 37-year-old, who has her own prize-winning label, will give her first show for Leonard next Monday.Leonard, which was founded by Daniel Tribouillard in 1958, and is known for its floral prints, has seen a succession of designers in recent years.