Great writeup! Loved reading it and adding this to my stash of known HA strategies.
Did you folks also chart out historical traffic and carry out this migration when it was the most sparse? Like early morning on a weekend or something.
Being a payments-processing company, we have a variety of users (marketplaces), each of whom has users who are widely geographically distributed, so our usage doesn't drop off as much as some other types of sites might on weekends. That being said, on weekends we see slightly lower traffic than during the weekday, so we performed this migration on a Saturday evening.