2.79 Noob question: copying and creating materials in Cycles As a beginner to Blender I wanted to practice working with materials. 
I made a cube and created two materials for the cube, one for the sides, one for top and bottom.
I repeated this twice (making cube, creating materials) and got annoyed with 


adding new material
selecting nodes output
deleting DiffuseBDSF and substituting with Principle
adding two Image Textures and assigning to them a texture and a normal map


So I thought to speed up things a little and decided to copy an existing material, with the goal of just deleting the OLD images linked to the Image Texture and substituting the NEW images.
And since all of my materials have the very similar name structure of material.png and material_n.png for texture and normal respectively, I had the even better idea to just type the new name over the old one in the Image Texture. Yes, I was too lazy to scroll through my 50+ materials.
(In every other dialog window, I can quick-jump while pressing T if looking for a file starting with T. Not in Blender?)

So, that way I learned that you can retype the image name in Image Texture, but obviously you are just changing a kind of "internal name", because the referenced file will still be the same. True?

The correct way to "recycle" a material may be:


click on "new", choose any existing material
click on "number of users" to make an unlinked copy
by clicking on the "open" tab in Image Texture choose your new texture.


Am I right in this? If not, what IS the correct way when you are trying to keep your build-up nodes and just want to change the linked files?

(And why do I now have a cube which insists of showing me the normal map in Texture View, even with having the correct texture linked to "Base Color" in the Principled BDSF?)

Any help would be very much appreciated
It seems that when typing the name it is referencing the previous same image with new name (not referencing external image).
So to refer external image you have to press open image and select the image.
Actually it very simple, Because blender had thumbnails display function, search filter function and many filters to show only specific formats and also options to show sub directory levels. So play with them they can be useful in many occasions.

All refereed images ( or old materials) can be selected (by browse button in left side ) till you close blender file. After closing blender file, Blender flushes all unused images (/unused materials have zero before name) and keep references to only used images(/materials). If you want to refer unused images later press fake user button. This is ubiquitous concept in all, like materials, particle systems, world materials, where ever you can browse.

You can also pack image ( image is saved with-in .blend file, it facilitates to use file in other than your system). Thus material is renderable even if original image destination is changed. But it increases blend file size and it is redundant (copies of same image). I hope these is help full for beginners.