How can I unverify a domain for use in another WAAD instance?

I have a test WAAD instance and a production WAAD instance.

I have previousy verified a custom domain with my test WAAD instance but I would now like to use that custom domain with my production WAAD instance.

1. The portal does not give me an option to unverify the domain in the test WAAD instance
2. Delete of the verified domain fails in the test WAAD instance ("Please try again. If the problem persists, contact supprt" - it does persist and I don't have a support contract)
3. I cannot verify the custom domain in the production WAAD instance - portal reports "Could not verify this domain because it was previously configured for your tenant or for another tenant"

Any ideas? Is this possible?

Thanks,

Andy