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I assume you are asking about the D for the R at the beginning. (Shortening by eliminating endings --as in Rich/Rick for Richard is a rather common way of forming nicknames.)

"Dick" was actually just one of many examples from Middle English of creating rhyming nicknames (arbitrarily changing the first letter). The forms Hick and Dick (appearing in writing around 1220) were at the beginning of a great 13th-14th century trend That's where we get all these surprising forms like Polly from Molly, Hob [and from it the last name Hobbes] and Bob from Rob (from Robert), Bill from Will (from William); and Hodge from Roger.

An interesting a variant on this is also preserved in the nursery rhyme "HICKORY, dickory, dock"!

http://www.straightdope.com/mailbag/mdick.html

http://en.wikipedia.org/wiki/Hick

For other forms of "letter swapping" to create nicknames, and a variety of other methods by which English nicknames developed see:

http://www.geocities.com/edgarbook/names/other/nic...

The most common substitute letters were H, B (sometimes the related sound P), and D (though I can't demonstrate how that came to be so).

But WHY did they do all this?

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!)