The $newnick identifier is filled when the ON NICK event is triggered on a channel. This event triggers when either a remote user changes their nickname on a channel, or the local mIRC client has changed its nickname.
Echo to the channel when the local nickname is changed, and halt mIRC's default message:
The example above should have an output resembling the following:
You are now ==> whoMe
Added: mIRC v2.1a