There is a company in Arizona that refurbishes battery packs for the Tesla Roadster. There are 2 options, a performance pack that reduces pack weight by 350 lbs. while sacrificing 20% range. Or, a long range pack that gives you 400 miles range. Tesla even makes an upgraded range pack for the Roadster.
I have also seen upgraded battery pack options available for the 1st gen Honda Insight.
Much like internal combustion cars, aftermarket performance parts are usually only geared to more expensive or enthusiast vehicles. In time, I could see battery upgrade options being more common.
https://www.youtube.com/watch?v=QFWCM6iJR_w