Always good to ask the question though. From a 100% environmental standpoint, it would be better to upgrade the battery when it's possible to do so than to buy a whole new automobile just for a different battery. At some point, all the Teslas on the road now are going to need new batteries. From what I've read, they are only projected to last about 300,000 miles on average - with many owners far exceeding this projection. The thing to do is to see what the options are at the time you decide your batteries simply don't meet your needs anymore. I'm guessing you will likely be able to a) buy exact replacement batteries brand new, b) buy rebuilt batteries that are just like your current ones except with new components (cheaper than new batteries), or maybe c) buy a battery with newer technology, which would mean either more range, lighter weight, or less expensive. Battery technology is advancing by leaps and bounds - like Moore's Law. I'm waiting on the technology to bring the price down for a battery for my home solar array.