Changes to lookup code/symbols on the NASDAQ?
You are tracking futures on natural gas. Therefore, the name changes because you look at contracts with different maturities: the first has a maturity in March 2016, whereas the second that you mention is due to expire next month (February 2016).

Future contracts are derivative which reflect accurately the expectations of market participants at a defined maturity. You should try to track indexes instead of futures if you want to track commodity prices without changing the tickers and without looking too much at expectations.