Up until now, if you wanted to play your current save file on Minecraft for Wii U, you were basically stuck playing it on the Wii U. Today, Nintendo fixed all that by announcing that you can officially transfer your data from the Wii U version over to the Nintendo Switch version. The update has actually made the transfer a pretty simple process. First, you must own the game on the Switch. Next, connect both devices to the internet and that they are both properly updated and have the correct account info on both. Next, go into the Wii U version, select your save file, and click "Transfer to Switch" followed by the prompts to get it transferred. That's it.
This is just another step of Minecraft and Nintendo to incorporate the game better with cross-play, as they already are making headway on their "Better Together" system that will allow you to use the same version of your game on other consoles and systems, with the exception of anything owned by Sony. Whenever that launches, it will be a big step forward in making it so every system can start experiencing cross-play, with the exception of some people still having their heads in the sand.