I was reading this technote.
According to the note, if you make a major version upgrade (from 4.0 to 4.2 for instance), you have to save all custom layouts and reaply it, but I don't understant why you need to perform step 4 (Click Reset Default next to the layout whose customisations need to be reapplied).
After the upgrade the new version's default layouts overwrite any customisation? or
Customisations are preserved and the idea is to merge them the new defaults? Step 5 says reapply customisations, not merge.
Thanks for your help.