The patch - which will require 61 MB of space on the Wii U and 325 MB on 3DS - will balance the characters in the game.
Once the patch is applied, existing replays will no longer be viewable, so be sure to backup any special ones you're particularly attached to before it arrives. Once the patch is available, it will be required to play the game online.
What changes do you hope to see in this update? Are there any fighters you hope will be adjusted? Let us know by posting a comment.