Is it safe to swap the "duckhead" connector from an old power supply to a new MagSafe 2?
The duckhead is very simple and it's safe to swap.

When you buy Apple World Travel Adapter Kit, you get various connectors that work with their whole range of AC Adapters.  There are not different models.

http://www.apple.com/shop/product/MD837AM/A/apple-world-travel-adapter-kit