Nintendo of Europe tweeted what you can do before and after the 3DS and Wii U eShop closures next month.
Importantly, Nintendo says you have until March 2024 to merge 3DS and Wii U eShop funds with your Nintendo Account.
Nintendo details how to use your balance before and after the eShop closures. You can use your Wii U and 3DS balance to buy Switch games by linking your Nintendo Network ID to your Nintendo Account.
After 27 March 2023, what can I do with my existing Nintendo eShop balance on Nintendo 3DS or Wii U?
If you have unused funds on Nintendo eShop for Nintendo 3DS or Wii U, linking your Nintendo Network ID to your Nintendo Account and merging your funds will allow you to continue to use those funds to purchase digital download titles, DLC, passes and other similar items for Nintendo Switch. The service for merging funds will remain available until March 2024.
If you haven’t merged your accounts, do so soon. We have instructions. Thankfully, March 2024 seems far away, but the eShops’ closure is near—time moves fast.