"Replace Original Type" What does it do?

[makigono1]

Hi folks. Anyone know what the "replace original type" button does?
Context:
I can set a trigger with the type "change unit type". When I do, I select a target unit, the type of unit I want to replace it with, and then there is a check box entitled "Replace Original Type". What does this checkbox do? It seems to have no effect at all. The implication of the button's title would be that it replaces all of the units of the target type with units of the new type, but this is not true.