After checking all the documents for each upgrade performed. I'm at the same spot. The error I'm facing is essentially while trying to actually remove the WebParts after Kentico v9.0 upgrade. I've also noted that after starting up the v9.0 upgraded code deployed for the first time the automatic procedure that should occur only once which will create the database objects and such is not running.
I'm also trying the new Kentico 10 install approach and move everything there, but I'm facing a different set of build errors on Visual Studio while converting the code to version 10. Given its 7 versions to go trough its basically the same thing as upgrading one version at a time because all the changes are listed incrementally by version, hence I got to upgrade the code one version at a time.