The FIFA World Cup Tournament poster is unique. There's only one for each edition. Once it's picked, it becomes the official image of the tournament. It can be designed (and picked) before or after the qualifiers. Before the TV/satellites or even internet era, the visuals used were (had to be!) produced long ahead.
For instance, the 2022 Qatar (20 November - 18 December) event poster was unveiled June 15th, 5 months before the tournament. It happened that some countries, like Russia in 2018, had an official poster and some different ones for the host cities. At the time, back in 1950, in a post-war world, communication was far from being as fast as today, and most of the visual communication was through posters and pictures.
From Early World Cup Posters 1934-1950 (emphasis mine):
The designer for the 1950 poster was chosen by a public competition, widely mediated in 1948, a collaboration between the World Cup organizing committee and the commission for the Brazilian society of arts, led by its President Mario Polo and a judging panel of Professors Castro Filho, Henrique Salvio, and Alberto Sims. One hundred and fourteen entries were reduced to a longlist of fourteen, with four finalists and a winner J. Ney Damasceno, from Rio de Janeiro. Damasceno won a prize of thirty seven thousand cruzeiros, although little is then subsequently known of him, and he does not seem to have been an established designer. So these processes of selection are in themselves interesting for how World Cup posters were chosen to represent the nation, not just by the respective football authorities, but also artistic experts.
There was definitely no reason to modify the official poster of the tournament.