Some systems will lock you out when downgrading from R9.0, leaving you with the DTE procedure as the only option for resetting security settings and gain access again. (DTE is ok if on-site, less convenient if you are doing it remote)
PS: If you are already locked out, upgrade back to R9.0 and do the procedure below.
Before starting the downgrade,
Add User: "DTEDefault" (similar to the NoUser)
Add Source Number: AT<SECURITYRESETALL>
Then open security settings and add:
Make it member of everything
When downgrading, tick all boxes in the main window (IPO + modules)
Un-tick "Backup system files", "Upload System files" and "Reset IP Phones".
Run the downgrade (upgrade) from Manager R7.0 or R8.x.
If you get error message "Some failed...", just continue, it will be fine.
When rebooted, login and transfer the SD card files from Manager, and remove "DTEDefault" user.
Login to Security settings and remove "Downgrade" user.