How do I change my UserId on Apple Developer Forums?
Yes, you are correct. 

You'll need to contact Apple Developer Account Support. However, in most cases you will get an answer as once your account is created, you will not be able to change your username that appeared when creating a username, and suggest registering a new Apple ID for the developer.

So, if you don't care, you can probably create a new user.

Or at least first give a try asking Apple Developer Account Support team.