According to the Q & A section of Wii Shop Channel on the Wii console, software developers can update already released Virtual Console games free of charge.
The section reads as follows:
The "Update" button indicates there are enhancements for the game. If you have already downloaded the game, you may select "Update" to download the enhancements free of charge. Please note, downloading the update may impact the saved game data.
Previously, President Satoru Iwata noted all of the expected plans in September, but never officially announced progressive updates. While Nintendo had noted there may be enhancements to existing games, no information specified updating games already available and purchased on the Virtual Console service.
Currently, it is unknown which games plan to offer updates. The Wiire will investigate this matter further, stay tuned.