ORIGINAL: Dr Honda
Even with my Tx's... I use a cycler of some kind when I charge... so I have a good idea of the capacity, and the general health of the pack. I voted Unknown since I change them when I notice a loss of capacity.
Replacing a perfectly good, and proven pack is actually less safe because you are now introducing an unproven pack that can have a manufacturing flaw just waiting to die.
As long as the cycler says that the battery is doing its job and holding a full charge, it stays in the plane. I have batteries that are over 6 years old and still hold full rated capacity