Add existing old ipad to new account

I have my ipad which was attached to an older email account.

I just upgraded to a new iphone which uses my current email.

I wanted to add the ipad to the "find my..."

But after I deleted the old email and tried to add the new on to the ipad, I get the pop-up;

"Email Used by Another Apple Account

This email address is not available

Choose a different address"

How do I add my ipad to this account?