There is no way to replace a class with a class in Symphony, need to redo the binding to the interface:
App\Service\S3Client\S3Client:
        arguments:
            $version: 'latest'
            $region: 'us-east-1'
            $host: '%env(MINIO_HOSTNAME)%'
            $port: '%env(MINIO_INTERNAL_PORT)%'
            $accessKey: '%env(MINIO_ACCESS_KEY)%'
            $secretKey: '%env(MINIO_SECRET_KEY)%'
    App\Service\S3Client\S3ClientInterface:
        class: App\Service\S3Client\S3Client
        arguments:
            $version: 'latest'
            $region: 'us-east-1'
            $host: '%env(MINIO_HOSTNAME)%'
            $port: '%env(MINIO_INTERNAL_PORT)%'
            $accessKey: '%env(MINIO_ACCESS_KEY)%'
            $secretKey: '%env(MINIO_SECRET_KEY)%'
when@test:
    services:
        App\Service\S3Client\S3ClientInterface:
            class: App\Service\S3Client\TestS3Client