We also migrated a while back (opbeat.com). While we run a smaller setup, I imagine that might be the case for most readers. We run a pretty standard setup with ELB, Postgres (master/replica), webservers and job processing servers.

This recipe details what we did (as i recall it):

  1) Prerequisites: Running at least two of everything in
     separate AZs and expertise (or courage) to fail over
     to a replica DB.
  2) Boot up instances of everything in the VPC
  3) Set up a new ELB inside the VPC, add the web servers inside to the ELB
  4) Make sure your instances inside the VPC can talk to
     which ever service they need outside (replica database
     and web servers needs to reach master outside +
     memcached). Use `telnet` to make absolutely sure :)
  5) Make sure web and job servers can reach the
     replica db and memcached inside
  6) Test out the new VPC ELB from outside
  7) Switch DNS over to the new VPC ELB, wait for it to
     propagate.
  8) Do a failover from your master to the replica inside.
  9) Same for memcached
  10) Shutdown everything in EC2 classic.
  11) Drinks

EDIT: formatting