RE: Batteries running-in and swollen batteries
If the bad cell is one of the outer cells, the replacemt is relatively simple. Otherwise, it is more involving.
Here are some obstacles to replace a cell.
1. The two tabs of a cell is soldered on to the PCB with lots of solder. A big iron is needed to heat it up for solder removal.
2. Cells can be damaged if too much heat is applied. The choice of a proper sized iron and the desoldering/soldering technique is criticle.
3. Possible of shorting a cell.
The saving probably is not that great, if the rest of the cells are near the end of life cycle.
I definitely not recommend going this route.