Your quote: The cost for the NEO upgrades on the A32x family rounds $1.5 billion...on the 737NG it's going to be a lot more expensive, they have to raise the LG, redesign the wing box, strength the wing box and wings, new pylon.
I don't think your observation regarding the re-engining of the Boeing 737 with the CFM56 LEAP engine is correct.
From Flightglobal: CFM's original timeline called for engine certification in 2016 and EIS with a replacement for the Boeing 737 or A320 in the 2018-2020 period. Boeing now talks about a 2020 replacement at the earliest, and Airbus now sets the date at 2024.
The new LEAP-X schedule makes it possible for Boeing to develop a "737RE" with an EIS around 2015-16, about the same time an A320RE might be available (see separate story).
Boeing has been cool to the prospect of re-engining the 737 because the Pratt & Whitney P1000G GTF diameter is too big. Boeing also has an exclusive supplier contract with CFM to power the 737, making selecting PW problematic in any case.
But with the news that CFM can meet a 2014 target date for a Comac test flight program and a 2016 EIS, this opens the door for a 737RE. The LEAP-X engine diameter is very close to the CFM56 now on the 737.
I know for fact the LEAP engine will be lighter than the CFM56-7 models used on the latest 737's with the composite fan and fan casing. All the changes you noted will not be required except for perhaps the pylon. I will bet the LEAP engine fan diameter will be the same as the current CFM56 engine come 2016.