An example of the basic scheduling behavior that you'll see when you change the Resource Units on a Fixed-duration task:
Your painter is assigned full-time (100%) to paint a room in 4 days. The work is 32 hours.
Then you assign the same painter another room to paint. Unfortunately the painter is committed elsewhere in 4 days and cannot work on the task beyond that. The painter can only work part-time (50%) painting the additional room.
When you enter 50%, Project keeps the duration fixed at 4 days, and recalculates work to be 16 hours.
Duration = Work ÷ Resource Units
Duration is the length of working time between the start and finish of a task. When you choose to make a task Fixed Duration, the task's duration remains at whatever value you enter and Project recalculates the resource units as you change assignments.
Note A summary task always has a Fixed-duration task type. A summary task is based on the earliest start date and the latest finish date of its subtasks, and the calculation for a summary task's duration is based on its subtasks. So, you cannot directly change duration for a summary task.