Okay, I had anticipated the message being more specific.

Once, GL8.1A is installed, make sure you have a backup of the GL8 data, then open the it data and run DIC. It may be there are integrity errors that are preventing the conversion to GL9 from completing. There is nothing wrong with running GL8.1A and GL9.x side by side though, only that once data has been successfully converted to GL9 you can no longer use the data with GL8.1A.