Before we start any AEM upgrades we should ensure that a detailed study is done on the release notes.
If the upgrades are planned to the next direct version (Say AEM 6.4 to AEM 6.5), We can just read the release notes of AEM 6.5 and proceed for the upgrade. But if the case is different (AEM 6.3 to AEM 6.5) ensure we are comparing the release notes for each versions.
For eg: Say we are upgrading from AEM 6.3 to AEM 6.5. We know there was an AEM 6.4 available. So while upgrade, first understand the release notes of AEM 6.4 and observe the changes between AEM 6.3 to AEM 6.4 and do the same comparison from AEM 6.4 to AEM 6.5. This process ensure that we are identifying every changes and accommodating all changes by taking precaution not to break anything during upgrades.
AEM 6.3 -> AEM 6.4(Release Notes) -> AEM 6.5 (Release Notes)
AEM 6.3 to AEM 6.5 Path Changes