The star destroyer is a pretty bad example. Replacing the engines on that would be so much easier than what would be needed for an aurora ship. But still, aurora vessels ought to be able to have their engines replaced without too much difficulty. And "getting rid of the old parts" is not something that I always want automated nor reliably an added cost. The part on the Star Destroyer tht is difficult to replace is the reactor, which is a very different case for vessels in Aurora. I mean, we have just gotten through talking about how most Vessels will have hordes of tiny reactors rather than one big one. The engines are, in all likelihood, going to be tubes. It just makes a lot of sense that way. Put in big bulges and you start losing space-efficiency and you start getting massive flaring and engine-wings and skirts and such to offset all your engine bulges. Make them flat and you have difficulty accelerating things and you need endless fields of surface-area. Make them boxy and they start having structural woes and efficiency flaws. Some bulbousness is to be expected, but really, it ought to be pretty easy to design them so that they can, with an admittedly large amount of effort, be yanked out the back with little harm to the vessel required. There really ought to be a recognition of just how different the components are. Replacing engines for engines should be relatively easy if they are the same size, and very difficult if they are of different sizes. Different technologies could conceivably require different linkages, but they all burn the same fuel and produce the same simple "thrust comes out here" effect, so that doesn't really seem like a thing. Replacing an independent turret with one that has the same weight should be easy, just pluck the bit off and put in another. One with less weight would be inefficient and one with more would require structural additions. You don't want to replace internal laser with railguns, lasers tend to produce more heat and railguns tend to produce more recoil, but replacing a laser of the same dimensions but with a different wavelength is probably more-or-less not so big of a deal.
Yes, in those instances where you have a massive internal unit, you will need to cut a hole out to replace them. Having the option to have prepared hull segments to ease this, at some cost to hull integrity but greatly reduced refit costs, i would like that option. In my experience it is often the case that the bulk of a vessel's mass is used in extremely numerous components. It is common for a vessel to have numerous, well, everything. The "replace the ships main powerplant" often just isn't a thing because there is no "main" powerplant and rather than a massive engine block sealed in a hull with a little shaft leading to a propeller, it is a massive great exhaust vent(Okay, admittedly it is probably a tiny exhaust vent with a particle accelerator behind it, but still, it is probably all a single panel that can be removed with less than usual fuss.)(Then again, the tiny vent is me being rational, the massive great exhaust vent on the other hand is what you will see in all the media...) that has a hole just the right size ready for it to be pulled out of.