do I really need to replace them if I'm re-using the same hose?
You might get away without doing so. However, the olive is supposed to deform to provide a seal, so technically you shouldn't. Might be fine though. Depends how tight Shimano's manufacturing tolerances are. Then again, getting the deformed olive off once it's deformed might be tricky, so probably worth trying it with the existing barb and olive first.