The best way to do this (on os x) is to use a command line tool, plutil.
It's used like this:
plutil -convert xml1 filenameToConvert -o outputFilename
These are the options for after '-convert': xml1, binary1,json. For this we'll use json. For this command you'll have to be in the same directory as the plist file. For this test we'll convert test.plist (an xml file created by xcode in xml format) into json. It will create a file named test.json:
plutil -convert json test.plist -o test.json
to convert it back we'd go:
plutil -convert xml1 test.json -o test.plist