Looks great. You've done a good job with the map itself, and it is always so enjoyable to see both daytime and nighttime version of a city map. For a setting like that, the nighttime version also really helps giving a proper look at the situation.

Change house layer should be able to be used at any time (in a city map). But it is only useful if you wish to use the demographics coloring of buildings in CD3, apart from that, it isn't really that helpful.

For the step by step:

1. Activate change house layer from the city meny
2. Select the buildings to change layer for (Finish selection with right click -> Do it as usual)
3. CC3+ now prompts you for the layer. You can type it in, but the easiest is to right click to bring up the dialog, pick the desired layer, and click ok.
4. All done

You'll need to change the width of the previews. Click the options button above the catalog and set the desired width.

That's what you say now.

That means it is an old map from before CC3+ (Or it was created from a flawed template that wasn't properly updated)

The default data file location was moved between CC3 and CC3+, and the # symbol refers to the installation directory, where the data files used to be kept. CC3+ has a fallback that it will be able to still display these files, but I guess the symbol import doesn't check this.

Fortunately, it is easy to fix, just run the FFIX command in your map to automatically convert the paths.