My girlfriend has just changed her name and I am suprised at how easy it is, I know in the UK it takes months and loads of paperwork, but in Thailand it takes 10 minutes.
So first off they have to decide on a new name, forename or surname or both, get them to check the spelling, make sure it doesn't sound like a rude word in some other language, not that there are rules against this but introducing your girlfriend to workmates or friends as "Miss knobgobbler" may cause a smirk or two, next nip down to the local amphoe / district office, show them your Thai id card and tabien ban, tell them the name you want, they will do a quick check to make sure of no arrest warrants, and hey presto the name is yours after paying the 50baht fee, now down to your local id card place, in Pattaya it is city hall and get your new id card.