Going, here, by memory of an old "Railfan and Railroad" article by Preston Cook... F2 and later F-units shared a basic structure, making upgrading a matter of changing "modules": for instance, the F2/F3 dynamic brake (with concealed blowers and two parallel rectangular openings on the roof), the early F7 dynamic brake (36" rooftop fan) and the late F7/F9 dynamic brake (48" rooftop fan) were all mounted on a removable "hatch" on the roof in front of the radiators, making change from one to another straigthforward. Note that not all upgrades would have been to the standard of a numbered later model: a railroad could choose, for example, to install the traction motors of a later model without changing the engine. (I think Western Pacific did this on a gradual basis: installing laterr traction motors on units as they came due for major overhauls.)