My neighbor missed the warranty, his 2011 went only down 3 bars before warranty expiry, while he wanted to keep driving his Leaf for another 5 years so he was contemplating when and how to shell out approx $7k for a new battery.
I asked him if he wanted an almost-new (late 2015) "Lizard" pack installed for $3k and he agreed that this would be a good deal.
Since BMS computer is paired to the vehicle, I had to remove all modules from his pack, bolt the original BMS to the Lizard cells and mount those into the battery, and bolt it back into his car. He is now enjoying 66 Ah and better-than-new range with his 2011.
It *is* possible to DIY by a knowledged person familiar with HV DC, if you can get your hands on a better set of modules.