If nick changing was not a big deal at all, then all forum softwares would allow users to change their nick at will (like signatures).
Changing nick also creates a mess in the forum archive. I think "tigers_eye" has been referred as "cats_eye" at least 100+ times in other people's quote/posts. It will confuse the new members and guests a lot, if they sometime browse the old threads.
Many people has been changing nicks since last year. I didn't like this, but didn't complain either because many of them wanted to use their real name or hasn't been in the forum for a long time.