Just registered mine with DTAC.. had same number for 20 odd years (I know we don't have to do it yet.. but I am off to the UK for a bit, and sods law says this change will be implemented while I am there)....
Dtac.. phone call centre get put through to an English speaker.. confirm name and number they send you an SMS with a link to register... on this link step by step instructions with pictures on procedure... pretty straight forward... but I would suggest doing it with someone who can read thai, as captions and some information fields are only in thai....