If you still have dormant funds in your Nintendo account on a 3DS or Wii U, you can merge your funds together so that you can share it with all your gaming devices. Make sure that you’re signed in on your Switch to your Nintendo Network ID before you proceed.
NOTE: Once you merge the funds of your different Nintendo devices, you can’t reverse the process. So before you proceed, make sure this is what you really want to do (and that you have permission, if you need it).
STEP 1: From the Home screen of your Nintendo Switch, select your Profile Page located at the upper-left corner of the screen.
STEP 2: Select User Settings.
STEP 3: Scroll down, then select Nintendo eShop Settings.
STEP 4: Select Add Funds.
STEP 5: Select Merge Funds from Nintendo 3DS/Wii U.
STEP 6: Next, select Merge Funds.
STEP 7: Enter your Nintendo account password.
STEP 8: Then finally, select Merge Funds.