The skins can be extracted using the Toolbox. Once extracted, you can unpack the skin files and access all the images. After making your changes, the images can be compressed back into a skin file and uploaded to the unit again through the Toolbox.
I’ve done this several times on both High and Standard MIB2 units. The key is to replace the images using the correct file names. For example, if you want to replace image 138 from a newer firmware with the prefacelift version from firmware 369, you may need to rename the file to match the image name expected by the newer firmware.
Simply copying all images from the 369 firmware skin into a 4xx or MHI2 skin will not work, because the number of images and their naming can differ between firmware versions. Unfortunately, the images are not all grouped together and are often scattered throughout the folder structure, so finding the correct replacements can take some time.
It can be a fairly time-consuming process, but after comparing enough files you’ll start to see a pattern. The corresponding images usually have numbers that are relatively close to each other, which helps when identifying the correct files to replace.
To obtain the prefacelift images, you can also extract the skin files from the firmware files. That way you don't have to obtain them from a car/unit with version 3xx.