Possibility of Upgrading Traction Battery in the Future?

That's hackery. People already replace the whole battery or individual cells, and the very definition of hacking is making something do something it wasn't designed to do.

Battery upgrading isn't likely to go mainstream for economic reasons (in 10 years, will you want a 33% EV range increase that costs as much as a used car in parts and takes a ton of custom labor that you have to figure out yourself, or will you prefer to put that cash towards a new self-driving Prius that already has the fancy battery in it?)

Obviously a few hackers will take option A, but most people will take option B.

The push towards the mainstream option is amplified by how long the batteries last in these vehicles, too. With expensive hardware, the bulk of the hacking doesn't start until the hardware goes out of warranty and then breaks.

 

@jacktheripper has been working on exactly what you are asking about. see his post
NexPower V3 hybrid battery unveil - Sodium-ion battery | Page 14 | PriusChat
or his priusapp.com web page

Jack has been hard at work testing and actually selling a LiFePO4 packs for Gen 2 and 3. Currently he has Sodium packs for Gen 2 and 3.

You'll see posts in this forum that either appreciate the work he's done so far (including me) and others that call his product snake oil.