In an ideal world it would only take one frame to switch resolutions, and even if the frame time is usually 16.66ms, it is sometimes faster than that. Since mostly what a monitor needs to do is resample the image from the source resolution to destination resolution, and possibly composite in the UI for adjusting the settings, it seems quite possible to implement. I'm sure if we actually looked at what all the layers are doing (X, video drivers, PCIe commands, etc) then we would probably be pretty horrified. Hacks and work-arounds and backwards compatibility at every level.