Depends a bit on how deformed the olive already is, I swapped a Deore brake lever once and didn't change the olive and it was no problem but if the olive is already very squashed it will be leaking.
You can't change the olive without cutting the old one off the hose. So it will become 1,5cm shorter or so. But if the olive that's one there now doesn't look too deformed you can get away with reusing it.
Depends a bit on how deformed the olive already is, I swapped a Deore brake lever once and didn't change the olive and it was no problem but if the olive is already very squashed it will be leaking.