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.
I dunno but exactly because of all the voices and considering the size of entire game I think that they might do a new download, they did same with the enhanced edition of DOS they did not patched original game but simply gave you new upgraded game.
Plus some patches can be abnormally huge, I still remember first time I installed WoW and then waited almost double the time of game download and installation for all updates to download and patch up.
And we can see from streams of the game from Swen that they did modified good part of the game plus not to count undead race too...
but at the end we will find out after 14th
