Most other GCP products can rely on their SDN to abstract away any differences between colo hardware. But this change is more a vertical through-hole change in the SDN itself — possibly involving, say, third-party network switches and middleboxes in their racks routing encapsulated SDN traffic. If those switches/middleboxes do enough in ASICs, they may have to literally do hardware swaps to execute this rollout.

Maybe a rollout thing, to make sure it's working well and avoid taking down all regions at once?

I wouldn't expect parity between GCP regions, since some of them are in Google data centers and others are in colo facilities. For example, GCP has a region europe-west3 in Frankfurt, but there is not a Google datacenter there. It's rented.

Rented or not doesn't matter. If you rent a rack or more in colo, you get your uplink fibres and starting there can use your own network equipment. And for big enough customers, the uplink fibres aren't necessarily ethernet but optionally MPLS, some patched dark fibre somewhere or whatever you like and want to pay for. So I don't see how not owning the building would make any difference.