People generally announce their name change if it’s just that they feel the old name no longer fits. Changing your name to disassociate yourself from your posting history doesn’t usually work too well for the above reason and the fact that people don’t really change all that much. People will soon notice that poster B acts a lot like poster A, connect that with poster A suddenly no longer posting and the gigs up.
I’ll be a bit vague here so as not to drag up old news, but at one time a poster that had been, let’s say controversial, had left the board (maybe banned, but I think just left) and later asked to come back under a new name or just made a sock. The poster was outed in no time. I can’t remember if someone noted similar posting styles or if someone the returning poster had trusted with the new name told other people, but soon the whole board knew about it. Bottom line, there are very few long lasting secrets here.