An interesting method, but as the comments say, getting your load balancer or proxy to do the right thing is the tip of the iceberg. Migrating your database on the fly requires schema version-aware code, facilities for split-brain upgrades (e.g. with mmm MySQL), sensible handling of in-progress requests. All that gets costly in terms of development time and complexity that, unless you're really big or run a critical-availability service (from hospital down to ad serving), is arguably better used making your product better. Just eat the few minutes of downtime after setting your customers' expectations properly.