Instructions
Remove your SD card from your Wii and insert it into the appropriate slot on your computer. If you don't have an SD slot on your computer you will need to buy an SD card reader, which usually isn't very expensive.
Click to open the SD card after the computer detects it. If you don't get a pop up, go to "My Computer" and find your SD card. Double click and open it.
Left-click to select and then drag the files you plan to move to the larger SD card. Store them in a folder on your desktop for now.
Remove the SD card and insert your new larger SD (or SDHC) card. Repeat Step 2 to open the SD card files.
Copy the desktop files to the larger SD card. Right-click and select "copy," go to the new SD card, right-click again in the new folder and select "paste."