Start, first, in the reference document section and look at the details of the Leaf battery box and interior layout. Then visit AESC and look at the modules themselves.
I import LiFePo4 cells from Taiwan, assemble battery packs, and build/wire/install management systems. What I'm going to say isn't guesses or theory.
The Leaf and Volt are using lithium manganese cells. Cell voltage should stay between 4.2 and 3.0V; 4.1V is better for longer life. Automakers are leaning toward these cells because like LiFePO4 they don't catch fire like laptop batteries when overcharged or damaged, and they're less expensive than other lithium chemistry cells.
One other type of cell that has the same voltage range and general capability is lithium polymer - like the ones used in radio controlled airplanes. LiPo has passed LiFePO4 in terms of performance level for the price today. The famous A123 Systems cells, for example, are high dollar in the LiFePO4 world and are 30C discharge. Current retail LiPo cells available from Taiwan are 45C constant and 90C burst discharge. That means a 100Ah A123 pack can provide 3000A for a short time - but the 100Ah LiPo pack can dump 4500A continuous and up to 9000A in bursts.
All lithium cells are coming down in price. Chemists and physicists are working on increasing the capability of anodes and cathodes. The largest jumps in performance will come from the sectors with the most focus - and for the EV world right now that's LiMn.
If need be, one can completely rebuild the Leaf pack to the cell level with LiMn cells that are made by a number of companies - including LG Chem. We can also rebuild the pack with LiPo and use it directly with no battery management system changes (provided we replace a 33Ah cell with 33Ah of LiPo) because they share the same voltagerange. Tomorrow's LiPo (LiMn for that matter) will likely have higher performance than today's LiMn.
Beyond those choices, if we choose replacement cells with a different voltage range (like LiFePo4's 3.6V to 2.1V) or the slope of the discharge graph changes, we can still build a replacement or add-on pack, but we'll have to either adjust the computer system (as easy as a firmware change or as difficult as building sensor converters to talk to the pack and translate into info the Leaf can work with.
The beauty of EVs is that we're not tied to any one cell type - there's always room to rebuild or replace the pack later.