I've always used OpenID to log into StackOverflow.
I see that it's going to be turned off in Feb 2014.
When I try to use an alternate means of login (e.g. Google) it appears to be trying to create a new account. I don't want that - I want to retain the one I've had for almost five years.
How do I switch my existing accounts to use Google to log in without creating a new account?