The client going from 32 to 64-bit means adding one binary file (the ESO.app itself), and any 3rd party libraries that it relies on (plus a launcher option about which binary to start). The bulk of the game's data, e.g. textures, meshes, sounds, etc. can stay the same.
But, since there will also be new content coming with the Thieves Guild DLC patch, this would have to be downloaded as well (the client always contains all that there is in the world of Tamriel, whether the corresponding DLCs have been bought or not). This might be a few GB, but nowhere near the client's current size.
There is a master copy of the client on a CDN, which is always the most current live version; the launcher ensures that it and the local copy are always in sync, either by downloading entire files (on a fresh install, or during a repair to correct detected mismatches), or differential files ("patches") to update local files to their new versions. Safety mechanisms like checksums normally ensure that the process of patching doesn't corrupt data -- in the event of it happening, a subsequent repair should fix it.
So, unless you see the usual signs of data corruption, like pink textures, save yourself the bandwidth of installing from scratch; the upcoming DLC patch will be big enough, again.