By default, task types are set to Fixed Units and when you simply assign a resource, by default they are assigned at 100%. So with these two variables in place, say you set up a task with 5 day duration and assign a resource. The tool automatically calculates the Work for that task to be 40 hours.
Given the scenario that you described, here is what I would recommend: I'm making the assumption that you have some tasks with more than one resource assigned.
-From Gantt Chart, Insert 2 columns, Effort Driven and Type
-Change the Effort Driven to No for all tasks
-Change Type to Fixed Duration for all tasks
-Now got to the Task Usage View (if only one resource is assigned per task, you can stay in the Gantt Chart view)
-Insert the Column Work
-You can now modify the Work for each resource per task to align with the effort that will actually be required to complete the task
With Type being set to Fixed Duration, when you modify the Work variable, your durations will remain the same.
Hope this helps!
J Black