Quote Originally Posted by Aussie2B View Post
It is still possible to add funds if you want, but I don't know the exact details of it. I've never made a Nintendo Network ID, but if you have one and link your system to it, you can add funds online or via Switch or whatever and then those funds will be available on your linked 3DS/Wii U as well. Sometime like that. If I'm getting any of the specifics wrong, feel free to correct me.
You have to (assume Nintendo doesn't just wipe balances) link the account to a Nintendo Network ID. Then create a profile on Switch, linked to the same Nintendo Network ID. Then on the Funds menu inside the Switch eShop, there will be an option to share the balance with Wii U/3DS. Again, hopefully Nintendo won't wipe balances and will keep them there to be reclaimed in this manner.
Problem if you are a user linked out-of-region accounts is that although Nintendo allowed me to use my US credit card to buy funds on the JP account on my JP 3DS previously, they don't allow that on Switch. It will detect and refuse CC or Paypal accounts outside the account's region. It'll let you still link the account, even if the Switch doesn't match, and you can even access the store. But to add funds, it'll only accept Prepaid Cards (which means you'll have to buy codes from a seller you trust).