Firstly, the poster 'bignok' didn't change his username. He simply stopped using the 'sparktrader' profile and created a new one for whatever reason.
It was very easy to spot - same abrupt, know-it-all style. Even Blind Freddy could see it.
Other posters have contacted support and asked to have their usernames changed. I forget which thread it was discussed in but apparently mods will do this upon request if you can make a valid case.
The only m instances I know of this happening are 'possum1931' changing to 'NoshowJones' and 'Bob Smith' to 'Bogs Smith' and back again.
The latter changes stood out because, IIRC, both users kept their old avatar. Also the old forum software used to show the reputation score so it was unusual to see a new username with an advanced score.
After that it was simply a matter of clicking on their profile and noting that both usernames appeared in the recent history.