Somehow, in setting up a new iMac, my AppStore ID got changed back to a defunct email address for my iPad. Mac and iPhone are ok but can't access updates on IPad and can't get a reset as old email address no longer functions. What to do?

Somehow, in setting up my new iMac and synchronising with existing devices, my AppStore ID got switched back to an old - and now defunct - email account, for my IPad ONLY. I'm not having any problem accessing the AppStore from the iMac or my iPhone, but I can't get updates or make new purchases via the iPad as I don't have the original password associated with the old email address and I can't get it reset as any reset emails will go to the defunct mailbox. I tried resetting via security questions but that only succeeded in resetting password for accessing store with new address on other devices. IPad AppStore access still demands old password And links to dead email address. Any ideas how to retrieve the situation? Thanx.