Yes, if you import a differently sized room background, you have to redraw all areas. Still better to do it now while you're starting out, I guess.
There's another way, though I'm not sure if it's worth the effort: you could import a white background of the same size, switch to editing the hotspots, move the transparency slider all the way to the left, then make a screenshot. If you crop and resize it, then import it using the "import mask from file" button, you'll just have to recolor all the hotspots, not redraw their shapes. Should work for walkable areas, walk-behinds and regions, too.