Here is a very useful utility that can replace file extensions for multiple files at once in a folder tree. It can be used in large software development projects where you need to convert files from one language type to another (i.e. C# to JAVA).
You can use it as you like. You can download both source and binaries.
System Requirements: x86, .NET 1.x or above, Window OS
Binaries - File Extension Rename Replace Utility Binaries.zip
Source - File Extension Rename Replace Utility Source.zip
Disclaimer: The software provided is as-is. Use it at your own risk. You are free to modify the code as you like.