Migrate frontdoor to another subscription without downtime, how?

Question

Frontdoor is currently in our on subscription, and we are required to move it. The resource does not support moves.

I am unable to configure the same cname / dns in the new frontdoor, as it required an active cname record to register, and that cname is already in use in the old frontdoor.

This means, that when i register it, i need to unregister it from the old frontdoor, which in turn means we will experience downtime.
Why is it not possible to register a frontend without actually registering the CName record? I would prefer to set up the frontdoor before i have to send traffic to it.
I cannot use an intermediate cname, as we use the domain name for routing requests, so it has to match.