People keep saying that's all you have to do, but it's just not true, you need to rename your SD directory accordingly. These next paragraphs are from
https://gbatemp.net/threads/3ds-sd-card-help.540666/
"Just copy paste everything from the old card to the new one except for the "Nintendo 3DS" folder. Once the files are transferred, go in the "Nintendo 3DS" folder on the old SD card, you will see a folder with a random name consisting of 32 characters (that's called the ID0 folder), inside that you will find another folder with a similarly random name (that's the ID1 folder). Copy the contents of the ID1 folder from the old SD card to the new one's ID1 (if there's no ID1 just turn the DS on and it will be created automatically, then transfer them over).
Basically, ID0 never changes and is part of the system-specific unique encryption key each 3DS has. ID1 on the other hand changes pretty much every time a new SD card is used among other times and the 3DS recognizes ONLY the most recently created one, which can make it so that sometimes just copy-pasting everything from one card to another as-is makes games inaccessible. The contents of the ID1 folder are anything you have installed, your saves (encrypted though), etc. and there should always only be 1 ID1 folder just to avoid troubles."
