I am just finished TCU upgrade ordeal in my recently purchased Leaf SV 2015 CPO. First of all, I have to find all relevant info and official Nissan bulletin to convince local dealer who actually was an early adopter of Leaf selling. Service reps have no clue when you talk about Leaf, all they know is oil change, tire rotation and inspections. Fortunately, the only "Leaf guy" was passing by and they asked him a question and shown the printed papers about the upgrade and he finally agreed it is free and he will do the upgrade. After upgrade nothing worked, portal was complaining about EV PIN and VIN mismatch and car was telling me to go to "Data communication menu settings". After 2 weeks of struggle and escalating the issue in EV portal, nothing was done at all, zero. So I found Leaf Navigation user manual and after reading through it, I stumbled over one page right after Carwings errors/troubleshooting page with bunch of screen shots and no explanations what it is for, but it was about Data communication. So luckily, I looked at this page and went to my car to see how it is configured. And it was set to "user settings" with manual provider On, but no provider chosen. I changed it to use automatic config and it displayed AT&T. And what do you know - everything started to work. So some how this setting was misconfigured on 2G unit, when 3G unit was installed - it copied all of those settings over to 3G from old 2G unit and left new 3G unit misconfigured.
Leaf 2015 SV CPO