Reversion? If it is for the religious procession that in principle only was by the Group of people who wore on, would be etymologically correct, except for the detail that does not exist. If it is by the verb to proceed, it would be improcesion, but it sounds far-fetched neologism (though there is already inadmissible). In any case, there is here a dictionary of antonyms little visited https: //www. meaning. org/antonym/procession. htm
