Proper way to reassign installed macOS to another Apple ID? I have a MacBook Pro (2009) which has seen it all from 10.5 and up to now where it is running 10.11 on an SSD drive.  I want to give this machine to my son, who has his own Apple ID.  

macOS is installed belonging to my Apple ID, which I want to transfer to his Apple ID. I created an admin account on the machine and married it to his Apple ID, and rebooted the machine with the “Reinstall current macOS version” keychord down. We got all the way through logging in, but then App Store complained that the macOS 10.11 download was not available to him. Repeating with my AppleID showed that the download was available to me.  

My theory is now that macOS is only available if you have “purchased” it (or if it was otherwise registered as eligible). I thought that this could be fixed simply with having a user on an eligible system, but apparently not.

What is the easiest way to reassign? Going back to 10.6 and then upgrading, sounds like a lot of work I'd like to avoid if possible.
You will need to erase the disk to change ownership, not just reinstall macOS over the current install.


  You should erase your startup disk in these circumstances:
  
  
  You’re selling or giving away your Mac, so you’re clearing your Mac of anything associated with you, your data, or your Apple ID.
  


To do that, boot into Recovery Mode by holding ⌘ cmd r during boot. Then follow Apple’s instructions for Erasing your startup disk and Reinstalling macOS:


  
  Select Disk Utility from the utilities window of macOS Recovery, then click Continue.
  Select your startup disk in the sidebar of Disk Utility.
  Select the disk name, not the volume name indented beneath
  it.
  Click the Erase button or tab.   If you don’t see an erase button or tab, select the volume name indented beneath the disk name,
  then look again.
  Complete these fields:
  
  
  Name: Enter a name for your disk, such as “Macintosh HD”.
  Format: Choose Mac OS Extended (Journaled).
  Scheme (if available): Choose GUID Partition Map.
  
  Click Erase to begin erasing your disk.
  When done, quit Disk Utility to return to the utilities window.
  Choose Reinstall macOS from the utilities window.
  Click Continue, then follow the onscreen instructions. You’ll be asked to select a startup disk and click Install.
  To verify eligibility to install macOS, your Mac serial number is sent to Apple, and you might be asked for your Apple
  ID.
  Your Mac restarts to a setup assistant after installation is complete.
  Complete the setup assistant with your son’s information.
  


Completing Setup Assistant is what will assign the OS license to the owner’s Apple ID.