I'm just following up on a recent thread about Mumsnetters who have been "outed" in real life. Lots of people said they had been, and even more said they had recognised people on Mumsnet, either from their unique stories, or from joining the dots between little pieces of personal info across multiple threads, or from photographs (e.g. of pets) that had also been shared on Facebook or elsewhere. Once you've been even vaguely recognised, your whole thread history can be located using Advanced Search, or even just by Googling the word Mumsnet along with the username.
It's easy to change your username. You just go to settings, overwrite the name, add your password and click save (you might get an error if it's not unique, so keep trying). Mumsnet rules say you can change the name as often as you like, so unless you really do want to build up a profile across multiple threads (fine if you do), why leave a trail for others to follow?