Thanks for your reply. I finally solved the issue.  Weird, because the Proseries files did not appear to exist anywhere on my computer, even when I did a complete search of the hard drive. However, I knew they were there, because I could open them in the 2019 Proseries program. Finally I opened a 2019 client's C corp file, closed it, found it as a "recent file" used, checked the properties of the file, which specified the location in the Basic program data file.  Back in the Proseries 2020 program, I changed the location of prior year files to the Basic data location, and was able to transfer the files to 2020.  Very strange that they were not discoverable through normal means. Anyway, whatever works, right?