The new identity stuff has much stricter password rules, they can be turned/tuned down but better to have stronger passwords
So, if your existing password isn't strong enough you will be forced to change it.
The first thing you must do is go to the Sign In page and sign in as normal, the new code will attempt to find your existing account and migrate it to the new identity providers for .NET core. If your current password satisfies the minimum password requirements then everything should go smoothly and you will be able to sign in.
If your current password does not satisfy the minimum requirements you will be redirected to the "Forgot Passsword" page where you can request a code to reset your password. You will receive an email with a link to reset your password.