Great question that is probably worthy of a longer discussion and it's own thread. In this thread and to answer it in this context -- I would say a rebuild requires young, controllable assets that grow together and peak at the same time to be a contender and then hopefully a winner.
Given than Boston does not currently have any blue chip prospects to build around and they are not going to terrible or drafting high any time soon, I would say it would be a long rebuild. Now, they could start trading off vets to acquire more draft picks and prospects and presumably start finishing lower and drafting a bit higher, but that doesn't appear to be the plan.
Re-tool might be the correct term. They are in transition from the Bergeron, Krejci and Marchand era to a new one. The issue might be that they had Pasta, McAvoy and great goaltending when they had those three...while they still have Pastrnak, McAvoy and Swayman they don't seem to have the next Bergeron, Krejci or Marchand in the system and I am not sure how you acquire those guys.