Currently when changereg is used on an online ADC user, his/her actual nickname is changed immediately, this is because ADC allows it. For NMDC that can't be done so then a reconnect is needed.
So it's not a bug you are seeing, it's a feature ;-)
But the reply from the changereg command on an ADC user can be improved, like:
Changed registered nickname from: [4Mbit]xxx to [1Mbit]xxxUser
The user is online, has been notified about the change and has been renamed immediately.