El Capitan was released September 30, 2015.
In good conscience, am unable to validate the above method issued by the Users will perform as expected.
I can only Validate the previous offered suggestions will perform as expected.
It would appear the user has, possibly, overlooked the benefits of Time Machine Backup over the past 6 years.
That, or, the personal data has not been considered as important enough to protect against Natural Disasters or Mechanical
Failures of the computer.
Good computer practices involves a Rescue Plan.
Do you have a Rescue Plan In-Place. This comprises having 3 Backups using 2 methods and 1 Off Site incase of Theft, Loss or Natural Disasters. A Time Machine Backup is very useful and can be used to Revert to Previous Working macOS.
For addition purposes - two Tested Bootable Clones each to separate external drives. This is insurance incase the upgrades goes sideways. At least one Tested Clone and / or Time Machine Backup should be Off - Site