Plenty of nouns change the second letter to become plural (man->men, goose->geese) but does anything change its first letter. I've hunted high and low over the internet, and spent ages browsing the questions at Oxford dictionaries but I can't find anything.
The only one I could find is cow/kine.
However, kine is mentioned as an archaic plural of cow in most dictionaries including OED but Wikipedia and Wiktionary mentions as regional or dialectal also.
Wordsmith does not count it as archaic and includes a contemporary usage:
Kine is one of the very few words in English (other examples: I/we, me/us) that have no letters in common with its singular form, cow. It is pluralized using the -n marker, as in the words children, brethren, and oxen.
"Cows stood belly deep in a ranch pond, doing their impersonation of the kine in John Constable's paintings." Verlyn Klinkenborg; Water and Grasses; The New York Times; Jul 5, 2010.
Interestingly, kine is a double plural also because an extra suffix has been added to Middle English plural form ki (ky) or kie (kye):
The word "cow" came via Anglo-Saxon cū (plural cȳ), from Common Indo-European gʷōus (genitive gʷowés) = "a bovine animal", compare Persian gâv, Sanskrit go-, Welsh buwch. The plural cȳ became ki or kie in Middle English, and an additional plural ending was often added, giving kine, kien, but also kies, kuin and others. This is the origin of the now archaic English plural of "kine". The Scots language singular is coo or cou, and the plural is "kye".