I would expect a Cloud implementation to be highly resilient. The technology for highly resilient infrastructure has been around for years. You can get any level of availability you want, at a price. That said, I would expect maintenance to be performed without service interruption by making use of those resilience facilities.
Even if you are completely replacing the infrastructure, it should not take 2 days to cut over to the new systems. I can't even imagine what their infrastructure looks like to need such an outage.