I recently looked back at a question that had an answer from a well known user and the answers is still there but instead of the old familiar moniker there us a user number. It seems he changed his handle Is that allowed?
That happens when a user deletes their account. See what EXACTLY happens when I ask to delete my account
Also, yes, we can change our screen name. I changed mine a while back from my full name to just my first name. Just go to your profile and click "Edit Profile & Settings" then you can change the name listed in the "Display name field". You are only allowed to change it once every 30 days.