If you reset your tama (press reset, don't download, but choose reset on the options) you will lose all your friends list, items/souvenirs, etc., generations information.... and you will have to re-enter your birthday info etc. - but you can change your user name that way.... this works on the V3, so I have no reason to suppose that you couldn't do the same thing on the V4.
I suppose it is just up to you if you want to start all over from scratch - just for the sake of a user name change