I've just completed a battery cell swap on my 2012 52000 mile gen 1 Leaf after the original battery was down to 74% SOH so I brought a 2013 battery of the later chemistry and just swapped the battery modules leaving the BMS and original bus bars and wiring in the original battery case.
So far so good! Its all working ok and I've been on a 40 odd mile run and the SOH, Ahr and Hx have slightly improved already.
I hope to keep updated with progress on this forum
I've put a few pics on facebook under scorpioelectronics/uk