This is not our first time upgrading AEM software. We have done numerous upgrades from AEM 5.6.x to AEM 6.1, AEM 6.2, and AEM 6.3 and from AEM 6.1/6.2 to AEM 6.3/6.4. The process and experience has never been this smooth!
We did in-place upgrade and follow the instructions provided by Adobe. We did a lot of readings and due diligence, of course.
We started first on our local environment, then moved to DEV (Development) environment. We moved to QA environment after completed on DEV. The product team verified the upgrade on QA, then we got sign off to move to Staging, and finally Production environment.
Our estimate is that there were approximately 40% customization on the AEM applications that we upgraded. Only about 5% of the features were broken. Compared to the past, it was approximately 70% of customization work was broken.
This is an amazing improvement done by the AEM Product Team.
By the way, we love Dispatcher 2.0.