Apparently ONE impetus in the letter swapping at that particular time was a dislike amongst the native English for the harsh Norman French "r". (Note how many nicknames made substitions for r's -- not only at the beginning of words [Richard, Robert], but in the middle or at the end of them -- Mary > Molly, Sarah > Sally/Sadie, Dorothy > Dolly [or just eliminating the R - "Dottie"]; Harold/Harry > Hal. (Though in this position the substition of L for R --a rather understandable one-- is most common, even hear you find the use of D as in "Sadie".)
My suggestion for the SPECIFIC letter changes is that SOME of them were based on sounds related to the original ones. (There is, for instance, a connection between the W and B sounds [of Will/Bill] --just note how you put your lips in almost the same position to say them. Similarly for M > P) Other nicknames could then formed by analogy with these forms, using the same letters. I don't know precisely how close "d" was to the Norman "r", but if we heard it that specific subsitution might make a lot of sense. (The use of "h" --Hick, Hob, Hodge-- almost sounds like an attempt to get rid of the initial sound entirely!)
My suggestion for the SPECIFIC letter changes is that SOME of them were based on sounds related to the original ones. (There is, for instance, a connection between the W and B sounds [of Will/Bill] --just note how you put your lips in almost the same position to say them. Similarly for M > P) Other nicknames could then formed by analogy with these forms, using the same letters. I don't know precisely how close "d" was to the Norman "r", but if we heard it that specific subsitution might make a lot of sense. (The use of "h" --Hick, Hob, Hodge-- almost sounds like an attempt to get rid of the initial sound entirely!)
