This article is from the Douglas Adams FAQ, by Nathan Hughes firstname.lastname@example.org with numerous contributions by others.
Okay, this is the footnote from the original radio scripts, and clears
up all this speculation and questioning of how it is everyone calls
Ford by his Earth name, and not by his Betelgeusing (sp?) one.
"Ford meeting Zaphod"
Many people have asked me angrily why it is that Zaphod Beeblebrox instantly
greets Ford _as_ Ford when I had stated quite clearly that he had only changed
his name to Ford Prefect when he came to Earth
It was very simple. Just before arriving he registered his new name
officially at the Galactic Nomenclaturoid Office, where they had the
technology to unpick his old name from the fabric of space/time and thread
the new one in its place, so that to all intents and purposes his name had
always been and would always be Ford Prefect. I included a footnote explaining
this in the first Hitch-Hiker book, but it was cut because it was so dull [DNA]
[Original Radio Series Scripts, p50]