MMDVer3 以降の .vmd から MMDVer2 で読み込める .vmd にする実験。必要ライブラリはコード内コメントを参照

Program.cs

// requires SlimDX (or any 3D mathmatics library) and Keystone.IO
// Keystone.IO: https://github.com/mfakane/Keystone/tree/master/Keystone.IO
using System.IO;
using Linearstar.Keystone.IO.MikuMikuDance;
using SlimDX;

namespace ConsoleApplication1
{
	class Program
	{
		static void Main(string[] args)
		{
			// TODO: change it to your vmd file path.
			ConvertCamera(@"X:\testCamera.vmd", @"X:\testCamera2.vmd");
			ConvertMotion(@"X:\testMotion.vmd", @"X:\testMotion2.vmd");
		}

		static void ConvertMotion(string inputFile, string outputFile)
		{
			using (var fs = File.OpenRead(inputFile))
			{
				var doc = VmdDocument.Parse(fs);

				doc.Version = VmdVersion.MMDVer2;
				doc.ModelName = "miku";

				foreach (var i in doc.BoneFrames)
					if (i.Name != "センター" && !i.Name.EndsWith("ＩＫ"))
						i.Position = new[] { 0f, 0, 0 };

				using (var ws = File.OpenWrite(outputFile))
					doc.Write(ws);
			}
		}

		static void ConvertCamera(string inputFile, string outputFile)
		{
			using (var fs = File.OpenRead(inputFile))
			{
				var doc = VmdDocument.Parse(fs);

				doc.Version = VmdVersion.MMDVer2;

				foreach (var i in doc.CameraFrames)
				{
					var rotation = i.Angle;
					var q = Quaternion.RotationYawPitchRoll(rotation[1], rotation[0], rotation[2]);
					var up = Vector3.Transform(-Vector3.UnitY, q);
					var offset = up * 6;

					i.Position[0] += offset.X;
					i.Position[1] += offset.Y;
					i.Position[2] += offset.Z;
					i.Radius *= 1 + i.FovInDegree / 45f;
				}

				using (var ws = File.OpenWrite(outputFile))
					doc.Write(ws);
			}
		}
	}
}