I have tried this 'perfect switch over' for a major domain after many months of planning. Everything worked fine for a month and then the traffic dropped to 30%

I have tried this 'perfect switch over' for a major domain after many months of planning. Everything worked fine for a month and then the traffic dropped to 30%. The traffic went back to normal level after few months. As usual there was no response from Google/ Forums etc. All I could speculate is the loss of backlinks. It takes months to reach the original level. Keep improving the site and you should be back and kicking in 6 months.