 League Swap/Superleague Guide
Uhh... before I start, I'd just like to say that this trick won't work with all leagues and sometimes the editor breaks things without explanation. I suggest you make plenty of backups and that you save and check(by starting a new game) that your database is in good working order at regular intervils if you are doing big edits.
The nation we are replacing is England. The nation we are replacing it with is Japan. Keep in mind, these are just examples; I know replacing England with Japan makes no sense at all before pointing that out.
1. Filter all English competitions and change their nation to Japan and continent to Asia. You may want to rename them as well. Do the same for the Japanese competitions, too.
2. Swap all the teams into the divisions you wish to put them into. DO NOT change the number of teams in any of the playable leagues, this will cause the game not to detect them on start up.
3. Set all clubs nations back to how they should be as the editor also swaps nation when you swap 2 teams. This is a problem with the editor.
4. Set all "last division" and "last positions" for the Japanese teams so that they were in the (former)English leagues last season. This is to avoid crashing when the game attempts to seed the FA Cup and League Cup.
5. Delete all competition history, then set at least last seasons history for the Premiership and FA Cup so the game does not crash when attempting to set up the Charity Shield. You can add as much history as you want for any competition, but this is optional.
6. Go to England and change it's TLA(Three Letter Abbreviation) to JPN, then go to Japan and set it's TLA to ENG.
7. See if your new league works!
Some of these steps(such as TLA and competition nation/continent) do not need to be done when making a super league, but you can if you want.
Thanks to Dave Fairbairn from Community.sigames for this | 
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