Is there a phonological reason for this change?
I know there are names where, when clipped, there is /r/ in coda position. For example:
- Derek > Der
- Sarah > Sar
- Harold > Har
So in non-rhotic varieties, these forms become Del, Sal/Sally, Harry/Hal/Hap to avoid the coda-position /r/. So is the change from /r/ in the cases of Robert and Richard, where /r/ is in a syllable onset, the subject of some phonological change or something different (e.g. Etymology).