You will have to retool anyway. The assembly robots will still need new programs to properly pick and place bolts, apply welds, etc.
Industrial robots are..... Very stupid. They run very literal, very exacting programs. Not much fuzzy logic in manufacturing.
Sintered metal (what aluminum 3D printing really is) also has structural limitations that solid machined metal does not have.
1) It has an amorphous/irregular/unpredictable grain direction
2) It has small voids in the resulting material
3) resulting metal grain structure has unpredictable grain size/orientation.
Combined, this makes the material have unpredictable strain and stress characteristics, making it less ideal for load bearing or structural components. The void problem makes the material more prone to stress fracture, especially from accoustic vibrations.
There's a reason why milled parts are milled. The designer of the part needed at least one of the following characteristics of milled metal:
1) Predictable grain direction (To mitigate internal stresses)
2) Uniform material composition
3) Controlled metal grain size and orientation.
Most CNC machines on a factory floor will have a tool carousel that has stock tools in it. (End mills, spot drills, face mills, etc.) The NC programmer will try to stick to these stock tools as much as is inhumanly possible, because the loading and unloading of custom tools into the tool carousel is where the infamous "Retooling" time comes in. As long as the actual tool numbers and tools remain constant between jobs, there is no retooling, even when making radically different parts.
If your assembly line keeps certain metal types in fixed work cells to maximize/economize workflow, then even having dissimilar metals in the production flow wont result in retooling. (Tools made for cutting aluminum are NOT the same as tools made for cutting steel, inconel, or other hard metals!! The cutting methodologies that are most efficient are VERY different! Tools made for steel will gum up when cutting aluminum, and tools made for aluminum will burn up/snap off when cutting steel. For this reason (as well as keeping waste streams seperate for recycling) dissimilar metals need to stay in different work cells, then converge at the assembly stage.
Other than producing shapes that physically cannot be either formed, or machined, (and castings are too inconsistent/inaccurate), I dont see a real niche for 3D printed metals. To me, the place they would shine would be in very novel shaped items that no tool in the universe can produce with subtractive manufacturing.
Say, things that purposefully have a large void inside them, or have some complex internal structure. (think corrugated cardboard, only made from aluminum)