B)1 3DS Method:
Before doing this, be sure that
Your banned 3DS did a System Transfer before, if not, this will not work.
So let's go:
1) Open Decrypt9 on your 3DS, go to SysNAND Options>System File Dump and do the two following procedures:
Dump LocalFriendCodeSeed_B and
Dump movable.sed;
2) Now open your SD root on your pc, and get the 2 files you just dumped in the files9 folder;
3) Open HxD (download it for yourself) and open both movable.sed and FCSL_B;
4) On movable.sed, copy everything except for the first 8 bytes and the last 40 (for dummys, use the indexes, you can start copying from row 00000000/ col 08 until row 00000110/ col 07);
5) Open LFCS_B, press ctrl + A, then paste your code (it must be and different code than before, if not, see step 7)
6) Now just go to Decrypt9 again and inject back the LFCS_B that you edited and you're good to go.
Troubleshooting:
7) If you did a system transfer before and your movable and LFSC are the same, you can do the following:
YOU NEED AN OLD NAND BACKUP TO PROCEED
8) Get your old NAND Backup and dump the movable.sed from it;
9) Check if the movable.sed seed is different from your actual localfriendcodeseed_b;
10) If it is, you're lucky, copy and paste it on your local friend code seed, restore your nand to your most recent one and then inject your new LFCS