No, H-2A workers are not allowed to move from one employer to another once they are in the United States on an H-2A visa. The H-2A program is designed to allow US agricultural employers to hire foreign workers temporarily when they cannot find sufficient American workers to fill available jobs. To ensure that US workers are not displaced by H-2A workers, the program requires that employers demonstrate that there are not enough US workers available to perform the work and that hiring H-2A workers will not adversely affect the wages and working conditions of US workers.
As a result, H-2A workers are typically hired for specific jobs with a specific employer, and their visa is tied to that employer. If an H-2A worker wants to change employers, they must first obtain permission from the US Department of Labor and go through a new H-2A application process with their new employer.
It's also worth noting that H-2A workers are only authorized to work for the employer who sponsors their visa. If an H-2A worker is found to be working for another employer without authorization, they could be subject to deportation and could be barred from entering the US in the future.