using System;
using System.Text.RegularExpressions;

public class Test
{
    public static void Main()
    {
            string json = "{\\\"test1\\\":{\\\"is_live\\\":\\\"true\\\",\\\"type\\\":\\\"default\\\",\\\"rendition\\\":{\\\"@name\\\":\\\"test\\\",\\\"@url\\\":\\\"mytest\\\"}}}";
            string json2 = Regex.Replace(json, "(?<=rendition\\\\\":)\\{(.*?)\\}\\}", "[{$1}]}");
            Console.WriteLine(json);
            Console.WriteLine(json2);
    }
}