Based on my experience changing the folder.svg icon :
First of all you can use a
folder.png file, rename it into
folder.svg and be done.
Second there is a folder.icon file in the
.../places/48 (default nautilus zoom is 48) that for some reason might index and cache the file for shorter loading times which I removed until I restarted nautilus to see changes applied to size 48.
Nautilus uses the appropriate icon size for the appropriate display size (zoom) of folders in nautilus seemingly to save loading times. So if you only change
.../scalable/folder.svg it will only apply to the "unused" big and tiny sizes. You will need to see which sizes are still the old icon and replace these too.
Warning: I think you need to kill nautilus and restart it to refresh changes and also it won't start anymore if it doesn't find a
This is the result for size 48 :