JSON covertion with key & value

Here is a sample program.

For clarity, I just manually created the source data with a class initialization.

The OP will mostly likely need to create the source data class instance with JsonConvert.DeserializeObject<sourceroot>(sourceJsonData) statement;

Here is a link to a tool that will create a C# class based on Json structures. It can be very helpful.

Code was built in Visual Studio 2019, and is a .Net 4.8 framework Console application.

using Newtonsoft.Json;
using System.Collections.Generic;

namespace CodeProject_JSonConverter
{
    class Program
    {
        static void Main(string[] args)
        {
            
            // This code block is to create sample data only. Use the JsonConvert.DeserializeObject<Root>(myJsonResponse) 
            // to initialize the sourceData class. 

            var sourceData = new SourceRoot
            {
                error = false,
                parameters = new List<SourceParameter>
                {
                    new SourceParameter {Name = "Vasanthakumar", FirstName = "Vasanth", LastName = "Kumar", Phone = "1234567890"}
                }
            };

            
            // This is the block of code that will convert the deserialized source class to the target class
            var targetData = new TargetRoot();
            targetData.parameters = new List<TargetParameter>();

            targetData.error = sourceData.error;
            foreach (var item in sourceData.parameters)
            {
                targetData.parameters.Add(new TargetParameter { key = nameof(SourceParameter.Name), value = item.Name });
                targetData.parameters.Add(new TargetParameter { key = nameof(SourceParameter.FirstName), value = item.FirstName });
                targetData.parameters.Add(new TargetParameter { key = nameof(SourceParameter.LastName), value = item.LastName });
                targetData.parameters.Add(new TargetParameter { key = nameof(SourceParameter.Phone), value = item.Phone });
            }

            // Serializes the target class into the new Json format
            var targetJson = JsonConvert.SerializeObject(targetData);
        }
    }

    public class SourceRoot
    {
        public bool error { get; set; }
        public List<SourceParameter> parameters { get; set; }
    }

    public class SourceParameter
    {
        public string Name { get; set; }
        public string FirstName { get; set; }
        public string LastName { get; set; }
        public string Phone { get; set; }
    }
    public class TargetRoot
    {
        public bool error { get; set; }
        public List<TargetParameter> parameters { get; set; }
    }

    public class TargetParameter
    {
        public string key { get; set; }
        public string value { get; set; }
    }
}