How can I delete an old iCloud ID on my mac?

Hi,

I changed email addresses around a year ago and have just noticed that my old email address is the default account name for my iCloud account on my iMac.

I'm locked out of making changes to this account -- I'm getting a message that it's been disabled for security reasons. Problem is, I can't log in at iforgot.apple.com because a) the email address is no longer working, so I can't access a change of password that way and b) the account doesn't seem to recognise my date of birth (clearly i must have entered it wrong when I first registered)

I'm registered with a different iCloud account on my iPhone, but I'd like to be able to sync the two devices. Is there any way of deleting the old account and changing it to my current one without going through iforgot.apple.com?

thanks

James