Let's start with a new jersey every season!
Step 1: make a copy of your game's DB folder and put it in the data folder for each season.
Step 2: import the teamkit list to the corresponding DB for each season.
Step 3: Delete the kit and kitnumbers folders from the data folder in the game and replace them with the same folders from the 10/11 season to Regenerate the game. So the jersey is 10/11 season.
If you want to change the shirt for the new season, just cover it with the data folder under the season 11/12 folder. New season only need to copy coverage, do not delete the previous. And can not be covered across the year, if you want to replace the 2012 jersey, you need to cover the 2011 jersey first.
when you want to go back to the 10/11 season and start a new game, you need to delete the files in the corresponding data folder again and copy the files related to the 10/11 season. And every time you change your jersey, remember to Regenerate your game.
Note that, just in case, you can make a backup of the same folder for your current game. Although I also provided the jersey file for the 10-11 season, it is somewhat different from the original.