Should people be able to change their usernames?

Interesting debate.

I'm currently building a web app that not only works using pseudonymous accounts but also supports and encourages the use of aliases (multiple profiles) to allow people to say things that they might not under their main profile.

I've run several very large communities in my time, and none of them suffer a lack of identity by allowing username changes. However policies that restrict changing username can have some bad consequences to it as it's not always obvious at the time of signup that so much data might be associated to you.

I'm personally of the opinion that you either allow changes, or you shift towards the consequences of real identity without the benefits of that (anti-spam and reputation).

Not allowing change means that people will work out your identity anyway, undermining any reason to have pseudonyms which can be very good protective barriers against the potential consequences (which on stack exchange is your employability).