You could not earn rewards for purchases or activity, nor could you exchange existing points for anything at all. I have a gazillion of those from playing Nintendo mobile games, but nothing to buy with them. No exact release date for the new program has been announced, but My Nintendo mentions that the change will be coming in early March. Besides purchasing games, you can earn My Nintendo points by using Nintendo apps like Super Mario Run or Animal Crossing: Pocket Camp on your smartphone and signing into Nintendo services like the eShop. So if a shopper does not have enough Gold Points to completely purchase a game from the Nintendo Switch eShop, they can use money in their Wallet or a credit card to complete the transaction. (We've asked Nintendo for confirmation that the same scheme will be available in the US.) Gold points are valid for a year, which means you can wait and accrue a sizeable stack of games before cashing in their associated points on a new title. Eligible games will offer gamers a 5% return on purchases.
Those Gold coins are about to pay off; literally. Assuming you have your My Nintendo account linked to your Switch, you'll be able to spend your coins when you're browsing the eShop on your console, paying the balance with any other payment method you prefer. While this seems to be the direction of speculation, Nintendo hasn't definitively set the value of a single Gold Coin.
Once earned, Gold Points remain valid for one calendar year, up through the last day of the month in which they were redeemed. So, if you earned them in March 2017, you'll have until the end of this March to use them before they expire.