Difference between command CPCFG and REBOOT VOLUME

 tmsh reboot volume 

reboots the system to the specified volume. The current volume saved config is not transferred during this process - the new volume boots with whatever config it had when it was shut down.

cpcfg

copies the saved configuration from a source volume to a destination volume (as long as the destination version is the same or newer than the source version).

K14724: Using the cpcfg command to copy a configuration from one boot location to another

By default, when you install a new version into a new partition, the current volume saved config is also transferred during the install. When you boot into the new volume, you will have the existing configuration (as it was when you installed the volume). You can control this behaviour:

K13438: Controlling configuration import when performing software installations (11.x - 12.x)

If you wish to install a version with no configuration into a new partition, I would recommend setting

LiveInstall.MoveConfig = disable

LiveInstall.SaveConfig = disable

From 11.5.0, when you select a new Boot Location in the GUI, you have the option to Install Configuration, which copies the latest saved configuration from the current volume into the target volume (if it is the same version or a newer version).

There are no supported tools that allow migration of a config from a newer version to an older version.