UPDATE (and background): Similar experience to you, Mutt.
My branch is in Minburi but we now live in Pra Nakorn, so I use the Khao San Road branch.
Anyway, turned up at the branch and was told that I had signed the relevant documentation in 2018, so no action required. Great .... however, because we are going back to the UK next week for the European summer, and it would be extremely difficult to rectify anything from there, I asked for written confirmation. Of course, no one was willing to put their name to anything, so I was told that they would double-check with Head Office and get back to me. Lo and behold, I get a call the next day saying that I do, in fact, need to complete a document!
TBF my situation is unusual. I don't use the branch where my account is held and the Khao San branch can only see a limited amount of data (why this is the case is another matter?). As you said previously, I get the impression that this initiative has been rolled out by KTB Head Office without much warning or training being given to the local branch staff.
Morale of the story: If you are a KTB customer, go to your branch asap. The process may take longer than a day to compete: I was told that the deadline for completion of the forms is 9 April. I have no idea what the consequences might be if this deadline is missed but best not to have to find out for yourself.
(PS: In case anyone is wondering why I don't transfer my account to the Khao San Road branch, the short answer is I can't for reasons I don't fully understand. Nor can I simply close my Minburi account and open a new one at the Khao San branch as I don't have a work permit (I'm here on a DTV which, apparently, doesn't allow you to open a new account🤷).