My Apple TV won’t let me change my tv provider

Use single sign-on with your device - Apple Support

“Change or sign out of your TV provider

If you change TV providers, you can remove your current single sign-on account information or set up a different one. Here's how:

1. On Apple TV, go to Settings﹥Account﹥TV provider.
2. Click or tap your the current TV provider.
3. Click or tap on Remove TV Provider.
4. Search for your new TV provider, then follow the steps to sign in.”

Possibly the key is to delete the old one first.

same tv provider but new user name

I haven’t seen any procedures for the situation that you describe. You may have to reset the device to be able to do that. (Is it common to get a new user name from your TV provider?)

Restore your Apple TV to its factory settings - Apple Support

What if we have the same tv provider but need to sign in with a our new user name. When I try it doesn’t work, the screen won’t come up to type in user name.