Halve the size of all of the sprites in the stage .sff and then manually reconfigure the positions of said sprites by changing the values in the stage .def.Afaik this is the only means of converting high-res stages to low-res, and it takes a lot of manual work (repositioning the sprites is basically trial and error, and you'll need to compare the two versions, which means you'll need to be able to run the high-res stage in the first place).
