This is a great question but I think it needs to be specified why the change in PM occurred. If it was due to that person leaving your company to take another job or personal reasons… etc. I believe the new PM should more or less ensure tasks are completed in the manner they were originally prescribed. However, this person will also be bringing a new perspective to the problem and might be able to make certain changes that can improve the project’s efficiency without making major changes to the scheduled tasks.

It sounds like your question may have been referring more to a scenario where there are major flaws appearing in a project causing the old PM to be fired or removed from the project. In this case, I believe it would be up to the new PM to decide if there are only portions of the current plan that will need adjustment or if the plan should be entirely redone. As you mentioned a factor that must be taken into consideration is how far into the project you are already along with how much of the project that’s been completed so far is actually useable. In either way, major changes should be expected and the failures experienced thus far likely caused some setbacks. The deadlines and expected timelines for each phase of the project would definitely need to be re-evaluated.