How to keep Windows 10
Now that I installed Windows 10, will I be notified when the time arrives that I can not return to Windows 7. Do I just delete the Windows.old directory to reclaim the disk space?
Yes, with the caveat that you go through the directory and make sure nothing you need is still there. You won't lose Windows 10 by keeping that folder, though. Windows 10 is on your computer to stay, unless you choose otherwise.
You may not be able to simply delete the Windows.old folder, you may need to use Disk Cleanup and tell it to clean up system files.
Once your W10 is activated you can do clean installs etc as many times as you want for THE LIFETIME of the computer you are using.
You can go backwards and forwards from W7 / W8 / W8.1 as many times as you like -- forget the windows.old file --just take backups using Macrium / Acronis etc.
Proviso: Your OLD Os must have been activated WITH LATEST UPDATES installed -- AND the INITIAL W10 install was an UPGRADE. After that you can clean install etc etc -- even wiping your entire HDD and re0installing W10 will still leave it activated (just SKIP when prompted for a key).
After July 2016 you will no longer get the free update.