I do not want to alienate my users, but having multiple ways to sign in to the site defeats the purpose of having an OpenID implementation. This is not a brand new site and has about 3k users which are somewhat diehards (we have a great community), but not all of them are and I do not want to lose anyone due to a sign in process.
Is there a simple way to convert the existing accounts into OpenId? or am I going to have to create some kind of "associate with old account".
To create OpenId's for each user you would need to offer your own identity provider service to have control over these users. And that might require more work than having two authentication ways. I suggest you offer both ways, it's how most sites handle this issue.