I see you have found them. On the PC they are in arduino-1.0.1\libraries\SoftwareSerial
You could copy the library to a new folder called something like MySoftwareSerial and then rename the relevant files within to match the folder name and edit the .cpp & .h files to also reflect these changes. Now when you download another version of Arduino you just copy the entire library into the new version and it *may work. I say may because they could have altered other files to break this version.