Yea I had to change my password, I thought it was something weird that I'd done..
This is a test post really, I couldn't use the main reply box a couple of days back, just the quick reply....here goes!
If you had an account here before the code was changed over to use .NET core.
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.
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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.