Extension Changer

Extension Changer is a lightweight and portable program which can turn the extensions of your files into something else, as the name implies.
Since this is a portable application, installing Extension Changer is not necessary. It means that you can store the app on a USB flash drive or any other removable device, and directly run its executable file on any computer.
The most important aspect is that no leftover items can be found on the hard drive or in the Windows Registry after deleting the tool. Moreover, you can always carry Extension Changer with you whenever you're on the go.
The interface of the app is based on a small window where there are instructions written. Upon initialization, Extension Changer automatically creates a folder named 'Process' in the same location.
All you have to do is copy the files whose extension you want to change to this location, specify the new format in the main application window (Extension Changer is not case sensitive), and press a button to get instant results. The program is able to process multiple files at the same time. It automatically overwrites the original files without giving you the possibility to create new ones, so make sure to create backups.
The simple-to-use program quickly applies changes to files while consuming a very low amount of CPU and system memory. It is very responsive to commands and worked smoothly during our tests, without freezing, crashing or displaying error dialogs. Thanks to its overall simplicity, first-time users can quickly figure out how to work with this app. Unfortunately, it has never been updated.