This article answers some frequently asked questions about the differences between the Manage by Project and Manage by Plans settings on a project.
Question: When would you move from one to the other?
Answer: If the desire is to ultimately manage by plan, it is sometimes advantageous to start off managing by project to ensure that your resources remain assigned to your project for the number of hours you will ultimately need them, even if you don't have a complete schedule (plan) worked out for them.
Please note that there is a nightly job that runs & affects projects - It resets the Start Date, End Date, estimated hours, and resources schedule(s) based on what is reflected in the plan.
Question: Are there any 'rules of thumb' for choosing one versus the other?
Answer: Managing by Plan allows the greatest amount of iterative management of the project. Additionally there are some projects that don't have a concrete end date and are prioritized as "side" projects. Sometimes it is advantageous to manage these by plan. Also projects that come from a requestor that you know, and cannot control a lot of scope creep (think Office of the President) can be managed by plan for example.
Question: Can you change settings in the middle of a project?
Answer: Yes, and the result of what would happen to cause a switch from managing by project to managing by plan I think is highly variable depending on your organization. Certainly the changing in a project's priority could cause such a change.
Question: What would happen to switch the other way? i.e. Plan to Project?
Answer: The system will capture all of the dates and schedules as currently shown and prevent further updates based on that nightly job.
Question: Nightly job? Is it possible to update my schedule changes before the nightly job runs.
Answer: Yes! Absolutely. Within the Resources section of the project, click Actions > Update Schedules to apply your schedule updates to the project.