First time installing dishwasher, elbow connection question

Just put in a new GE dishwasher to replace an old one. First time I've done this and I've gotten to the point where it said get a 90 degree elbow with ferrule and compression nut. Now when I look at the hot water hose that's already there it's about 3/8" hard rubber with the end connecting to the elbow being molded with a nut already on it that can't be removed. Can I just install the new elbow and reuse the old nut? The elbow I bought has a small brass piece that goes in the end where the nut connects, but I can't put it in with the old hose. Is that absolutely necessary or will I have any problems if I don't put it in?

Replace all the old parts. If you have to cut a bit into the wall, do it. It will save you the hassle of leaks or other repairs later.

hose

If the old connection fits the new elbow you can reuse it, but a better way would be to replace the entire hose if it is rubber or plastic.