I'm not an expert on this, but I've got a fairly good background in the field, and from what I know, you can't do what you want.
In very brief summary, cell balancing means drawing charge out of one cell until the voltage on that cell is the same as all of the others. With >100 cells in the Leaf pack, there are a lot of cells being individually discharged to get the pack balanced. You don't just swap two wires to the total string and have it balance.
Also, I believe that the BMS keeps track of battery information. I've read that replacing the battery in a Leaf is more than just swapping. There is some programming or configuration loading as well. If that's true, you can't just swap packs. You need to keep the BMS with the pack and swap both.
Bob