Asset post processor to change .NET framework version from 3.5 to 4. and possibly mark the projects as unsafe.

ProjectFileMod.cs

//#define ALLOW_UNSAFE //Uncomment this if you need to be in unsafe mode

using System.IO;
using UnityEngine;
using UnityEditor;
using System.Collections;
using System.Text.RegularExpressions;

class ProjectFileMod : AssetPostprocessor
{
	private static void OnGeneratedCSProjectFiles ()
	{
		foreach (var filePath in Directory.GetFiles(Directory.GetCurrentDirectory(), "*.csproj"))
			UpgradeTargetFrameworkVersion (filePath);
	}

	private static void UpgradeTargetFrameworkVersion (string filePath)
	{
		string content = File.ReadAllText (filePath);
		string searchString = "<TargetFrameworkVersion>v3.5</TargetFrameworkVersion>";
		bool changed = false;

		if (content.IndexOf (searchString) != -1) {
			content = Regex.Replace (content, searchString, "<TargetFrameworkVersion>v4.0</TargetFrameworkVersion>");
			changed = true;
		}

		#if ALLOW_UNSAFE
		string append = "</DefineConstants>"; //HACK, find this location to inject the AllowUnsafeBlock
		string allowString = "<AllowUnsafeBlocks>true</AllowUnsafeBlocks>";
		if (content.IndexOf (append) != -1 && content.IndexOf (allowString) == -1) {
			content = Regex.Replace (content, append, append + "\n\t" + allowString);
			changed = true;
		}
		#endif

		if (changed) {
			File.WriteAllText (filePath, content);
		}
	}
}