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Maybe. As time goes on and they're used, the pack capacity will diminish. Some cells will degrade faster than others. So, say you only have 5 cells and the capacitys are as shown after 5 years:
1) 80%
2) 84%
3) 74%
4) 79%
5) 82%
Obviously, the 74% one is the most degraded. You can replace it with a new 100% cell. However, now your weakest cell is 79%. So, you gained 5% more capacity which isn't a ton. If you want to get back to more than 84% capacity, you have to replace all the cells. This is most likely what will happen. Darin showed this when he capacity tested his Insight cells.
The other way things could go, if they are poorly designed or just have low quality batteries is one or two cells could just go bad much faster than the others:
1) 50%
2) 84%
3) 74%
4) 79%
5) 82%
Now, obviously the 50% cell would be replaced. Now you go from a pack capacity of 50% to 74% which is a huge gain. This case is much less likely IMO as OEMs have tested stuff to death and we really shouldn't have these issues. However, on DIY stuff, with using cheaper components and no BMS system sometimes, this is more likely.
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