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.

I have never had the need to run a repair or even re-install the client as far as I can remember, and my first and only download of it was at its initial release during the stress test beta. On my main machine, that is. Maybe I'm just lucky, but there are a few factors that play into it, I think (see below). Anyway, regarding the running of regular repairs, I'm inclined to follow the old programmer's credo: if it ain't broke, don't fix it.

MMOs are games that constantly need to load data from disk into memory, not just during loading screens, as they simply can't hold everything at once. By their very nature, they are undetermined regarding what has to be displayed from one second to the next, as player movement and gear are random. Magnetic HDDs have the slowest random access times in the system by a full order of magnitude, i.e. they are slower by a factor of 1000 when compared to RAM. Files that get changed or added to a lot might even fragment, i.e. get strewn across non-consecutive file system blocks, to the further detriment of seek times. This can be mitigated by a full re-install, or preferably, by the use of SSDs.

But this isn't the only factor, I guess. I suppose that a bit of performance loss over time comes from factors like addons and their saved data, the shader cache growing, etc. My first approach to this would be deleting/renaming the live or liveeu folders in Documents/The Elder Scrolls Online, and let the game build a fresh one from scratch, to see if this changes anything.

I also made it a routine to reset the graphics system to its defaults, by pressing [X] in the video settings, after each major patch. Not only get initial settings set to different defaults from time to time, but I've seen frame rates go up, or at least go back to their values before the patch.