The number of Hierarchy Levels does not change for existing Strategic Plans when you update the number of levels in Settings.
Changing the default number of levels in Settings won't affect existing plans. This preserves the data from existing plans, while still allowing the change to new plans.
To update the levels for an existing Plan, you will need to first delete all Objectives in the plan (Which also deletes any Initiatives and Linked Investments associated with the Objective), then you can change the level for the Plan by clicking the edit icon in the Summary section of the Plan.
Note: You can use the Edit icon in the Plan's summary section to view the plan properties, including the number of levels the Strategic Plan currently allows.