Because of the many differences between the 1.X and 2.X series FUDforum, there is a special script you need to use to upgrade. This upgrade script will convert from version 1.2.8 to 2.0.2.
It is very important that you make sure that your old forum is running version 1.2.8, other wise the upgrade script will fail. If you see that your forum is older, you will first need to upgrade to version 1.2.8 before continuing to upgrade to 2.0.2.
This upgrade script is available from the FUDforum's download page in 3 compression formats: bzip2, gzip and zip.
The links to download this script can be found under the 'Upgrade from 1.2.8 to FUDforum 2.0.2' sub-heading.
Now that you have the upgrade script on your server, decompress it and place it inside your forum's main web directory, so that you can access the upgrade script by going to http://you_forum_url/upgrade.php
Now that the upgrade script has been positioned, open the http://you_forum_url/upgrade.php URL with your web browser, this will begin the execution of the upgrade script. The script will output various progress indicators, letting you know of what is it doing at the moment.
If you have output buffering enabled or using buffering modules such as mod_gzip you may not see any output of the upgrade script until it finishes.
Upon the completion of the upgrade script, you will need to run a consistency checker to rebuilt various caches and verify consistency of your forum's data. The upgrade script will try to automatically to run the consistency checker by popping up a new window using JavaScript. If for some reason such window does not appear click on the 'MUST click here' link, which will execute the consistency checker in the current window.
Once the consistency checker finishes running, the upgrade process is complete. The only thing left to do, is to remove the upgrade.php script you've placed inside the forum's web directory in the 1st step.
You should also consider upgrading from 2.0.2 to the later stable release if one is available by following instructions in the Upgrading 2.X and 1.9X series FUDforum section of the manual.