How to upgrade ThinOS correctly

As I got more and more request on how to upgrade ThinOS device the correct way, I thought it would be a good idea to write a small summary.
I will not explain how to upgrade the clients technically using a fileserver with /wnos folder, etc. This is something I assume you all know.

ThinOS is designed to upgrade or downgrade from any version to any version. However there are a few exceptions:
1) Units with new Dell Mac Address ranges require 8.1_027 or higher code. In newer 8.4_108 code there is actually blocking to prevent downgrades.
2) Due to file system changes it is recommended, you upgrade older units to from 6.x / 7.x to 8.0_512 prior to upgrading to 8.1/8.2/8.3 or 8.4.
3) It is recommended you upgrade directly from 8.0_512 to 8.3 or 8.4. There are some known issues where upgrades to 8.1 or 8.2 will not unpack code correctly and result in errors (example
ICA EXEC errors).
4) When upgrading to 8.3 or higher you are now required to set a global Security Policy (SecurityPolicy=) in the INI file or a security level of Warning will be applied to WDM and File
Server SSL connections.
5) When downgrading from 8.4 you are now required to disable the Digital Signature check (Autoload=value VerifySignature=No). Digital Signatures are a new security feature to protect code
integrity in 8.4 and newer code, that and these are not found in older image files.

Please note this also applies to ThinOS Lite (for Citrix, also known as Xenith) where 1.x Xenith = 7.x ThinOS versions, and 2.x Xenith = 8.x ThinOS versions.