I don’t think you can change it based on setup, field help says:

Factor Unit
Use the list box to select one of these values:

Hours/Unit - How many hours it takes to manufacture a unit.
Units/Hour - How many units can be manufactured in an hour. 
Hours - How many hours manufacturing takes, regardless of the number of units.

This field is mandatory and the default value is Hours/Unit.

Likely the only way would be a custom event to flip it to what you want on the addition of an Operation line.

Thanks Shawn,

I have seen it flipped behind the scenes but was not sure how to do that.  Thanks for your suggestion of the custom event also.