Sometimes I’m in a situation where I need to handle very high resolution layers in Photoshop and would love to work with smaller resolution proxies instead and only later have the high detail image for the corresponding layers. But, I’ve never figured out the way to replace the contents of the Smart Objects (proxies) without the layer dimensions exploding on the main composition after the file inside the Smart Object has been resized. Well, now I finally found a way and I’m so happy about it that I need to tell the world! (I really couldn’t find anything from the web.. maybe it is too simple and obvious task? )
This is really simple. When inside the Smart Object layer file, do not change the pixel size straight from the pixel values, change the resolution value instead!! The important part here is the Document Size values must not change. Then just bring the high resolution version on top of the image (flattening might be a good idea too) and save. When you go back to the main composition, the dimensions of the layer are exactly the same than before but the low res layer is now high res. And of course, if you made any duplicate layers of the Smart Object they are updated too.
I can’t believe I didn’t find this out sooner!