How do I change my userid on the Apple Developer Forums? As shown in the below screenshot, it seems there is no way to change this. Do I need to simply call Apple support?
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.