Who (other than myself) can view my OpenID?
If I change my OpenID (add new one, then remove old one), does StackExchange keep a record of the removed OpenID in database?
Moderators (those of us with ♦'s) are the only other users who can see your Open ID.
Previous OpenIDs and alternate OpenIDs you have associated with your account are logged. Moderators can see this information from the detailed user history if they look, but it is not shown on your profile page.