I saw a teenager on YouTube replace the battery on an early LEAF with a newer, higher capacity pack from a later model and (separately) a Chinese firm offering replacement packs for the same car at capacities never offered by the OEM.
Notably the kid said the hardest thing about the swap was sourcing the replacement battery.
I really think given a few more years when millions of Model 3s/Ys reach their 2nd/3rd owners, we will see this become more routine.
Also people talk about pack replacements like they are a single monolith but many cars have packs composed of 10-20 modules. Some modules can even be repaired at the cell level. So costs can trend down over time.
The trend has been towards unrepairable monolithic packs. You could do module replacement in the model S packs but since model 3 it's all full of foam.
Swapping for a used battery is pretty easy if you have a lift. Far easy than swapping an ICE engine. It makes sense that there's more aftermarket batteries in China as they have far more EVs and willingness to tinker. We in the US have become cultural allergic to tinkering, everyone cries that you're voiding the warranty or it's too dangerous.
Notably the kid said the hardest thing about the swap was sourcing the replacement battery.