I would expect that each authority server's configuration data is securely and reliably backed up, so if the physical servers are seized, then a replacement can be fairly easily provisioned. Maybe some operators will even have one waiting on standby, in case of a normal hardware failure or similar.
Presumably the IP address space isn't being seized or otherwise disbanded, so it could be dropped in exactly as specified in the hard-coded configuration - that is, the same IP address and port.
Any ongoing and persistent impersonation of the server wouldn't be viable, as the long-term directory authority identity keys are kept offline.
Maybe I'm missing something here but it sounds like more of a symbolic violation, rather than a potentially catastrophic disabling event that brings down Tor.
I think you're probably overestimating the portability of IP addresses, and underestimating the coercive power of the organization that does the seize.
IP addresses are assigned to a network provider, not a customer. If I'm hosted by a US provider and the FBI seizes my server, is that provider going to say "sure, just spin up a new box on the old IP", or are they going to tell me to get off their network? What if the FBI has an opinion on which decision the provider should take? What if the FBI has an opinion on which action the directory operator should take?
Can they really say to MIT, for example, that port 9131 on IP address 128.31.0.39 is now out of bounds? Or coerce all of Riseup's peers (including any future ones) to cease connecting to their entire 199.254.238.0/24 range, or even just firewall off 199.254.238.52 upstream?
As far as I know, there's no precedent for this where the provider - and, crucially, the owner of the IP address block - is hosting the server itself.
I would expect that each authority server's configuration data is securely and reliably backed up, so if the physical servers are seized, then a replacement can be fairly easily provisioned. Maybe some operators will even have one waiting on standby, in case of a normal hardware failure or similar.
Presumably the IP address space isn't being seized or otherwise disbanded, so it could be dropped in exactly as specified in the hard-coded configuration - that is, the same IP address and port.
Any ongoing and persistent impersonation of the server wouldn't be viable, as the long-term directory authority identity keys are kept offline.
Maybe I'm missing something here but it sounds like more of a symbolic violation, rather than a potentially catastrophic disabling event that brings down Tor.