1. I downloaded the json to csv converter
2. I expoted the league to csv
3. I edited the player names, Team IDs to put the players on the correct teams, Experience and Ages; nothing else
4. I converted the CSV back into a json file
5. I successfully import the json file back into the game with no errors

The player names successfully changed, but the teams for the players did not. If this import does not work, I will need to move hundreds of players to their correct team manually.

Did I do something wrong? Thanks!

The import won't change teams.. it only edits players. However, if you use that file to create a new league it will pick up the edited team id.