Some of the software is mission critical but not flight critical. If your targeting pod is down, it'll be harder to see the ground target or visually ID aircraft from afar, but there is no safety issue to the plane.

For example the aircraft is unstable by design and software keeps it responding and making changes N times a second (IIRC 40times/sec or something like that) without which it might crash - so that system would be one you don't want to change frequently.