Being Honest to Helps
A good message from a vendor, such as Evernote, is key in explaining downtime to users.
Personally, after having done migrations like this in the past, I would have opted to do a live migration where the data is replicated over a period of time to the new servers and there is a hot failover for any issues. This ensures that there is no downtime, although in the worst case it might cause intermitted service this can be solved by ensuring by failing gracefully.
And naturally 15-20 minutes for a migration of a key part of the infrastructure is attainable in only the best case scenario’s and at least a day should be taken to ensure that any issues are resolved.