I have no idea how they accomplished that with the Monkey Island remasters or the other more recent titles. In Visionaire the only realistic way for you to do that would be to create 2 separate scenes & change between them, however I don't believe it's possible to change the min/mag filter or enable/disable the "activate pixel effect" option during game runtime - unless Simon has made that possible at some point.
It's kind of complicated as the full-hd version of your game would be best displayed using "linear interpolation" filter, whereas the pixelart version would be best displayed using "nearest neighbor" as that's what keeps the pixels sharp instead of them being blurred around the edges to make them look smoother.