I also say don't replace just one cell in a battery pack.

The other old cells were probably form the same manufacturer's lot as the one that failed. and they have had the same amount of use. They will be very likely to fail soon. May as well replace them all.

Why trust your model to a debateable condition battery?

Hmm... try to save $20 to $30... and risk $300 to $3000 worth of airplane. Does that sound like a good plan?

You can go to Wal-Mart, get 8 AA NiMH cells (1800 mah) for appx $18... and solder up a TX pack if you want to play with soldering on a battery. (it works well... but you HAVE to know what you are doing to get good joints!) I am right now operating my TX and RX on battery packs made this way... but I have a friend who is VERY good at soldering up battery packs and does a professional quality job of it. (I don't trust my solder joints on batteries.... and I do pretty well at soldering... Think about it.)