Non-destructive File/Folder Replace
Select the folder you wish to copy.
Press command + C. This will copy the folder to clipboard. 
At the destination, press command + V. This will copy the folder to your current folder. If a folder with the same name exists, you will be give options to Stop, Merge or Replace.


Quirk: This doesn’t work if you try to move (by dragging), not copy, a folder from one place to another. However, if you still wish to use dragging and dropping, drag with the option key pressed. This will copy, instead of move, a file/folder from one place to another.

This is what you will see if you copy a folder named “Temp” to a folder named “untitled folder” which already contains a folder named “Temp”.