2.0 configurations CANNOT BE DOWNGRADED
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A word of caution here, the configuration of a 2.0 platform is not compatible with the 1.2 platform. It also not possible to downgrade from a 2.0 release to a 1.2 release.
So before testing a upgrade:
SAVE YOUR CONFIGURATION FIRST!
Then perform the upgrade. When you want to go back to 1.2 you will need to reinstall and restore the configuration.
My least effort 1.2 system restore guide says to save your 1.2 configuration on a MSDOS formatted USB1 USB stick. You need to have a directory in the root called "conf". Then store the configuration file as "config.xml". So the USB contents will look like "E:\conf\config.xml".
Then boot from the 1.2 installer CD, it will automatically load the correct configuration from the USB stick and proceed as normal with a working configuration. You can now install to hard disk with option 99. During the install the configuration will automatically be copied into place. So after the reboot your system will function as it was before.