Same can be done under file preferences from TUI or you store the preferences in your home directory (folder starting with .fluentconf)https://forum.ansys.com/discussion/comment/112723#Comment_112723

@DrAmine, it's still not working nI used /preferences/general/default-ioformat 'Legacy' TUI command, and I get the following error.nSo I gave up and used Ansys 2019R3 which worked finally, but now I get the following error which stops the simulation before it starts.
'Error: Update_Time_Level: invalid datanError: Update_Time_Level: invalid datanError Object: #f'