Hi there,
According to http://en.wikipedia.org/wiki/APK_%28file_format%29, APK format is a variant of JAR format. As per my understanding everythign that contains in a JAR format is compiled version. A Jar file has its own Signature Key when is generated at the time of compiling. So when you rename APK to any other extension, its quite likely that the key is changed. If you modify the Jar file, you need to compile it again and then install it in your Phone.
Maybe you wanna try this?
cheers