Here's how to keep receiving .esd's for future previews:
(from Windows 10 forums)

If having the possibility to make an ISO image and create your own install media weighs more than reduced download size, you must manually disable UUP in Windows registry.

Both of the following DWORDs have by default value 1 (UUP enabled). You must change both values to 0 (UUP disabled) to keep getting the ESD file and full build download:

Key: HKEY_LOCAL_MACHINE>SOFTWARE>Microsoft>Windows>CurrentVersion>WindowsUpdate>Orchestrator
DWORD: EnableUUPScan

Key: HKEY_LOCAL_MACHINE>SOFTWARE>Microsoft>WindowsUpdate
DWORD: SupportsUUP

These DWORDS must have the same value, 1 for UUP enabled, 0 for UUP disabled.

If you have already upgraded when UUP was enabled, you do not have and can't get the ESD file. Do the above registry changes before next build upgrade to assure you will get ESD file.

If your upgrade has been downloaded and prepared but you have not yet clicked Restart now button in Windows Update to start upgrade, you can do the following to redownload upgrade and get the ESD file:
- Edit the registry as told above
- Open Disk Clean-up > Clean up system files
- Select Temporary Windows installation files, click OK:


- Wait until clean up is done
- In Settings app > Windows Update, click Restart now
- Windows will now restart for upgrade but as no upgrade files will be found, it boots back to desktop
- Now let Windows Update to download the upgrade again, this time it will be the full upgrade and include the ESD file

Thanks for fellow member @Bat 1 for registry tip.