I have got a Layer in Gimp with an image that I manually scaled down, many times I want to just replace that image with a different image file but with exactly the same coordinates and size. Basically I want to change the source/url of that image while maintaining the size and position.
There's nothing in GIMP that will do it automatically.
Crop the new image to the same aspect ratio, and then scale it to the same size as the one you are replacing. Select All, and Copy.
Back in the other image, select the image layer you wish to replace, and Select All. Edit > Paste in Place. Layer > To New Layer.