The thing about the Galaxy class (and to at least some extent, probably the Nebula as well) is that it already has a lot of 'modular' space built into the saucer for expansion or mission-specific configurations.
It seems more likely to me that the Nebula is less a 'swappable module' ship and more just a common, cheaper design that's easily produced in different subtypes that are best suited to certain roles. There's also the Melbourne type which has two miniature warp nacelles up there! These various types don't seem very swappable to me. Look at the module pylons for the Phoenix vs the pylons for the Farragut - they're totally different. I'm not so sure the Nebula is quite as adaptable as advertised. When they went to CGI for the Nebula class, however, the modelers basically just kitbashed the Galaxy parts at 1:1, substantially increasing the volume of the ship. The engineering section is also subtly different in shape, and certain details (like the nacelle pylon phaser strips) appear proportionally larger than on the Galaxy class. I think there are minor details on the saucer that are also supposed to suggest this. The Nebula was originally conceived of as being significantly smaller than the Galaxy, which I think is kind of suggested by both The Wounded and First Contact.
