It is not unusual for an unvoiced |ch| to be corrupted to a voiced |j| sound. There is further support in that Chauncey is a variation of Chance, which rhymes with the |Jance| part of the name. |-y| in both cases would likely be the familiar/diminutive form of the names.In naming cultures that are based, primarily, on euphony, there is no compulsion to maintain spelling and pronunciation standards.See LATHAN for a more extreme example.Admittedly, it IS a SWAG.* Feel free to offer other/better ideas...*Scientific, Wildly Assumptive Guess...