We just recently did a 62kwh swap into a 2012 LEAF at the shop where I work. It went pretty well, although there area few hiccups to doing it in a 2011-2012. The main thing is that the SOC and "Guess-O-Meter" will not show the right info most of the time, and we also had a couple issues with DCFC (which should be fixable with a couple tweaks if the customer decides to bring the car back, they didn't seem too concerned with the ability to fast charge).
On a 2013 or newer LEAF this swap should go over pretty much without a hitch. Smaller batteries can also be swapped into 2011-12 without any issues. The cost to do a battery swap is approx. $2,500, plus the cost of the battery itself. This cost includes the communication module, adapter parts, and all labor required for the swap. We can assist in finding a battery, or you can bring your own. Battery pricing can vary depending on the source, but to give ballpark figures, a 62kwh pack costs around $8-10k, 40kwh about $6-8k, 30kwh $4-6k, etc.
Here's a quick YouTube video we put together of the swap: