Usually, project managers are assigned to a project from the beginning to the end. This strategy has its advantages as continuity and strenght relationships with stakeholders.
However, there are other ways to assign project managers.
A program manager should consider assigning a more senior PM for the kick-off or initiation phase of a project to ensure a good beginning. Once the project is defined, the goals are approved and the baseline is established, you can execute the project with a more junior PM.
To perform the transition at the start point of the execution phase is something easy if you have all the information agreed from the initiation phase (project definition, baseline…).
Close projects is too a critical stage in projects, some projects needs a more senior profile to close the project and avoid the closing phase to be exceeded for a long time (with bad economic consequences for the project). The transition in this phase to a new PM is more difficult at this phase of the project, but an additional support to the junior PM can be enough to ensure a right close of the project.
Do you have any other strategy?
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