If you are changing the duration, then you don't want Fixed Duration. Perhaps something here will help you:

• Effort-driven does not affect what happens when a variable (such as Duration) is changed; it only influences what happens when you add or remove resources.
• Fixed Units: if you change Duration, Project will recalculate Work. If you change Work, Project will recalculate Duration.
• Fixed Units, Effort-driven: If you add an additional resource, Project will recalculate Duration.
• Fixed Units, NOT Effort-driven, If you add an additional resource, Project will recalculate Work.

Remember, nothing is recalculated the first time you add resources.

I suspect you want Fixed Units, NOT Effort-driven. This way if you change Duration, Project will recalculate Work. And if you add resources, Project will recalculate Work.