On Fixed Work task, work will be fixed. If you change the units, duration will be changed.
This means that increasing Duration or Units (Resources) to the task will not affect the Work.
Tam and Megan works 8 hours per day from Monday - Friday; Task Type is Fixed Work. If you have task with 4 day duaration and task has been scheduled for 32 hours.
Later, if you increase the duration of the task from 2 days to 4 days, Project sets each resource's assignment units to 50% so that each resource works only half-time and work remains constant at 32 hours.