For the sake of completeness: it seems to depend on the packet forwarder that is used.
When using a forwarder that enforces a specific gateway id (like based on the MAC address), then indeed one should never delete it. This applies to, e.g., the legacy forwarder, which is also used when registering the TTIG on V2 (though that uses Basic Station). But other forwarders allow for choosing one’s own gateway id. For those, one simply needs to choose a different unique id when deleting and adding it again. From Slack:
@proffalken 2020-03-09 3:32 PM
Hmmm, I guess my gateways don’t tend to force the eui as the ID - I’ve got both Multitech and RAK gateways that have been added/removed from various TTN Accounts, so YMMV
Same might go for devices: their unique id might be blocked for future usage, but as you can choose whatever you want, that seems fine. Beware:
(More details in that post.)
I don’t understand the message that is shown for “change owner”, so I’d be on the safe side and always add the new owner as a collaborator. See Is it possible to change the owner of a gateway - #6 by arjanvanb.