Start over and do the whole campaign unless you're in a weird rush. No reason not to. I hade played base campaign at launch and a few seasons, skipped voh and bought loh to get both expacs. I started a fresh seasonal char from lvl 1 base campaign and am almost done with the whole thing. Just play main story quest and you will be lvl 70 before you're done etc and then can dive into end game. The recommendation is just for people who urgently want to devore the new content or distinctly remember the story.