To progress the wider migration from the old row-wide ASWs in codfw to the new EVPN-based devices, we need to move the IP gateway for existing vlans from CR routers to Spine switches.
Unfortunately, while interruption can be minimised, experience from moving the public vlans has shown that minor interruption to comms for some servers is to be expected. Hosts will not experience an interruption on IPv6, but it's likely that some will have an interruption on IPv4 traffic of between 10-60 seconds.
Codfw is currently our primary site, but we can't wait until we switch back to complete the change. It probably does make sense to depool the site in DNS for font-end connections, however.
UPDATE: In the end we waiting until after the DC switchover to complete this, as it was deemed to risky. It was completed without disruption, however, see below wikitech page describing the approach taken:
https://wikitech.wikimedia.org/wiki/Migrate_from_VC_switch_stack_to_EVPN