The Windows.old folder contains the files from previous OS or version, and is used when the user wants to rollback to the previous OS or Windows 10 version. This folder automatically clears in ~30 days after you upgrade to Windows 10.
If the folder was created when you installed version 1607 (Anniversary Update), the folder might be removed automatically in 10 days.
If Windows doesn't clear the Windows.old
folder for some reason, you may remove it it manually using Disk Cleanup (cleanmgr.exe
- run as administrator), and clearing Previous Windows installation
files.
Recovering data from Windows.old
If you find some of your settings are lost after upgrade, you should be able to recover the program specific data files from Windows.old (ProgramData and AppData folders). Move everything you need, to a different folder outside Windows.old before clearing it out.
The Redstone upgrade counts as a major upgrade, just like the 1511 upgrade. – Daniel B – 2016-09-25T09:31:50.297