Based on what we know module by module swapping is the only DIY route possible, which is obviously more labor intensive. New modules are different than those in 2011/12 cars, so they don't just bolt on, some additional effort will be required to install them. It can be easier to move the battery controller instead.
Finding a complete battery by itself for 2-3k may turn out to be problematic, as if someone take it off they are likely to sell it as individuals modules, currently they go for $100 each give or take, 48 of them in the pack, you do the math.
Sure, if someone buys a wrecked Leaf for a DIY battery swap the numbers should work out well considering they can sell other take off parts, but this is too much of a project for most Leaf owners out there.
'11 SL, totaled
-1CB@33k/21mo, -2CB@53k/33mo, -3CB@68k/41mo, -4CB(41.5AHr)@79k/49mo, -5CB(38.85AHr)@87.5k/54mo
53.92AHr, SOH 84.5%, 140k miles9kW Solar