Customizations should be maintained when a full release (10.4 to 10.5) is installed.
This concerns for example
- customizations of the GUI (menu, toolsets, floating windows etc.)
- settings regarding scrolling behaviour and shifting tolerance
- folder links
- WMS-Server saved in ArcMap
It means a high effort, to re-establish an approved working environment that supports you best at your field of work at each major release.
At the moment, the user is notified of this in the install process with the following message if the previous version is still installed: ArcGIS x.x for Desktop have been detected and will automatically be replaced with ArcGIS Desktop x.x. Any software customizations, such as extensions or add-ins must be re-applied with their x.x versions after completing set-up."
The idea is to store customizations in a file before updating and to apply this file after having updated to a major release.
See also Esri Case #01926143 .