So I'm not happy with my ESO account UserID (the name) but I can't find any way to change it. There's no option for it on the account page nor on the support page. I tried sending an email but it gave me a generic "Oh, you forgot your UserID?" response.

y u not happy with Longdong90 ?

i would like to know that too

If you have additional email accounts you could always create a new one. Assuming you haven't eg. already bought the game.

Failing that maybe try contact customer support.

Best to talk with ZOS tech support:

https://help.elderscrollsonline.com/app/home?region=US

Unfortunately, I already bought the game. I tried the customer support route as far as email goes as well. Trying the phone option next.

I emailed and asked them, they didn't say yes or no but asked for some of my details like username and some security questions so I imagine they'll change it.

This is the correct answer. We can't help you here with this.