Why does your site keep insisting I use the old choir username and password?

It’s not me or my site that is “insisting” that you use the old login details, rather it’s your browser. Modern browsers try to be helpful and remember the logins used for every site that is visited, so it is remembering the one used when you last visited my site.
On the login screen, your browser has helpfully (?) entered the Username and Password you last used.
This is easy to sort out. The next time you login, click into the Username box and delete the password that your browser has entered for you, and enter your new one (which is usually your email address). Similarly, click into the Password field and delete what you see there (usually a series of xxxxxx), and enter the password you chose.
Before clicking on the Login button, check the Remember me box. The next time you visit my site your browser will offer you the new login details.
A more drastic approach would be to clear the browser’s cache, but then it would wipe out logins for all the other sites you visit.