My guess is only a patch. But it will be a very big patch. Most likely the file structure of the game will change to support mods and we have the ENTIRE voice over library to download which are usually quite large. My guess is the only things we will not have to download are textures which have not changed from EA to release, the rest of the game files will probably need to be downloaded again.
Take this with a grain of salt because I have 0 clue how this will actually go down, other than I know we have to download all the VO files which are going to be several gigs.