There is a new video titled “Remastering Day of the Tentacle and Grim Fandango” in which some DoubleFine developers explain how the high resolution graphics were created and how the developers wrote a wrapper around the old SCUMM engine to use it with the updated graphics/sounds.
I suggest watching this video to anyone interested about the SCUMM engine.
Since I have read a lot of people suggesting that the remastered graphics of DotT were created with some upscaling filter, here is the exact point of the video where they debunk this myth and show how the graphics were re-drawn on Photoshop: