You have to consider that in German the K in front of an N at the beginning of a word is always pronounced. At one time it was also pronounced in English but as English evolved the spelling stayed the same but the K became silent. One example is the word knife. As German immigrants and their descendants in America became more Anglicized they also Anglicized the pronunciation.
Here in Pennsylvania some people still pronounce Knauss with a K sound while for others it is silent. So to answer your question the K was there all along and probably some descendants decided to spell it more like it sounded.