It should be relatively easy to ditch an online identity. Start with changing your email (no mail forwarding either), then set up new accounts for the services you care about.

I changed my identity on HN and reddit all the time -- just create a new account. Of course these services could probably connect new with old just by IP address, but I don't care about that. What I'm actually doing is gaurding against something coming back to bite me in the ass years later (an unpopular opinion, lapse in judgement, etc).

Cahnging legal stuff, that depends on your country.