I'm sorry but statements like that freak me out a bit.
Do I get a warning when my instance is going to be migrated instead of resized, and an estimate of the downtime?
Because unless you found a way to defy physics you are pretty surely not migrating e.g. a 4GB Ram/40GB disk instance in anywhere close to 2 seconds.
If you had all of the instance data stored on a fast SAN, so that it was available to all of the hosts simultaneously (possible.)
And you had an ultra-high speed interconnect (40Gbit Infiniband would do) between hosts, for sharing the memory state when you migrate..
4GB of memory, at 40Gbit/s would be transferred in 0.8s. (assuming perfect throughput, all cows are spherical, etc..)