I had forgotten, till I noticed this, that he was originally known as William the Bastard.
Click to enlarge. Detail of a roundel of William the Conqueror ('William Bastard'), from a genealogical chronicle of the kings of England, England (East Anglia?), c. 1340–1342: British Library Royal MS 14 B VI, membrane 5.
That is a pretty dramatic, and positive, switch in personal brands. How many other examples are there of such positive switches in soubriquets?