Cannot change primary email address

My primary email address is no longer in operation so I wanted to convert my secondary email address to primary and add a new secondary address.

I deleted my secondary address then typed it ito the  Change Primary Address box  but got the message "email address already used".

Tried again after half an hour and was still getting the message. I have currently got the same address for primary and secondary, but it is one I don't use much.

Any ideas please?