For architectural scale models, you almost have to start from scratch or rebuild them using the existing model as a guide for the scale you want to print at. The reason is because when you scale down an existing model, you're left with tons of features that are too small to print, so you have to start over and exaggerate the size of features that are important to show, decide what needs to be left out. It's pretty easy to repeat after you've done one, because the modeling is so much simpler. I guess an analogy would be low-poly models for faraway components in a game vs. the high detail closeup objects.