This thread has covered all sorts of name changes, many of which are due to phonetic alteration or language borrowing over the course of time.
However, the original question is about quite a different thing. There is no phonetic or cross-language aspect to the alteration of initial consonants. This is an odd method of making nicknames that was quite prevalent in Middle English. (I haven't any explanation for it.) Some of these no longer exist except in surnames (ending in -s or -son, e.g. Hicks, Hobson), but to repeat those already mentioned and add a few more:
Richard = Dick = Hick
Roger = Hodge
Robert = Bob = Hob
William = Bill
Margaret = Peg = Poll
Helen = Nell
Oliver = Noll
Edward = Ned = Ted
Three odd things are that the H alternants have all disappeared; and that not all common names got these forms; and that you couldn't just ring the changes on them: I don't think William ever got called Hill, Nill, Dill, or Pill.
