Just wanted to highlight for this thread that veteran players will have a choice between levelling the new way or the old way. Of course the level numbers will be different but the OP here will be able to do what they want to do as it currently stands.
IMPORTANT: This is future content and is subject to change at any point in time. I can only comment on what was mentioned at Blizzcon 2019 by the developers. Stay tuned to your fansite of choice for datamined entries or participate in the beta for Shadowlands if you are lucky enough to be selected for it 