What if Ignacio Zaragoza had been killed in action at the Battle of Puebla? His triumph there and subsequent campaign paved the way for Mexico's victory in the Second Franco-Mexican War and began Mexico's rise as a Great Power under Juarez's and his presidencies. His death could well have resulted in a defeat and domination of the country by France. Also, a French victory could have averted or delayed the collapse of Napoleon III's monarchy.
Even had Zaragoza had been killed in the battle, it is unlikely Lorencez could have triumphed. The Mexican defense was too well planned, and Lorencez forces would be soon outnumbered by arriving Mexican forces.
An early victory would have been very hard for the French to follow up on (and Lorencez was, simply put, the wrong man for the job), and the expeditionary force as it was composed in spring 1862 was in serious need of reinforcement as it was thanks to yellow fever.
The real turning point came during the siege in 1863 when the cream of the Mexican army was trapped in Puebla and forced to surrender, leaving the way to Mexico City open, cost the Mexicans valuable supplies, and forced Juarez to adopt a policy of guerrilla warfare and retreat to the interior which did not change substantially until American intervention in 1866.
If the French siege of 1863 is defeated then that is a brutal turning point for the French intervention, and if 28,000 Frenchmen and their Mexican allies can't take Puebla and the way to Mexico City, then the intervention is essentially doomed. The French are confined to the coast where they will wither away from disease, and Juarez remains safe in Mexico City with most of his nations wealth and a powerful field force at his disposal.