Just did a quick check in the game. It looks like you can export the roster from a standard game to a text file. Then edit the teams for each player in the text file and reimport. This may or may not be faster than manually changing each player.
Game settings/league settings has the export and import buttons. I have never tried this so I don't know if there are quirks you need work arounds for. Maybe guys in the historical forum can help since I think this is something they would do more. I have done historical gaming in over 2 maybe more than 3 years so I am rusty.
Manual section on this.
https://manuals.ootpdevelopments.com...port_functions