By 'previous internet usernames' do you mean 'old email addresses' ? Email addresses are used to create the things you listed such as forum accounts, edit Wikipedia, join Yahoo groups etc.
If they are email addresses from old ISP's, I wonder if they can be recycled once you terminate your contract and another user signs up? I mean, if my name was common, such as John Smith, and I formerly had
John.Smith@telstra.com, would another John Smith be able to take my old address if I switched ISP?
I guess for all important accounts that you sign up with email, it is important to update your details when switching ISP.