Its not particularly easy. One of the reason I frequently reused sections of my world map for different projects is just because it took so damn long to redraw everything at scale initially and then rescaling is a pain. Your best bet is to double the scale to try and go from a 1 pixel border to a 2-3 pixel border when it rescales. When a raster graphics file like the one tripleA needs goes up or down in size, the shape of the line will change because you can’t recolor/fade the line to preserve the shape. The map can still function and there are ways to clean stuff up using map skins/details. See any of the tripleA maps based on Domination to get a sense for what that looks like. The map in that case will still usually still look reasonably good if players are running at a 50% zoom, though sometimes text can get harder to read and it may get some fuzz at different scales.
Otherwise you may be able to do more in Photoshop or Gimp by isolating the color (in this case black or white/blue) and expanding or isolating by 1 pixel. But you will almost certainly have to redraw something going that route, since it will give you gaps or chunky spots.
Basically this is one of the things that holds tripleA back as a game engine. There’s a lot you can do in tripleA, bit drawing/rescaling baselines is kinda stuck with what it is.
You might ask Frostion or Hepps about the tricks they use with map details to polish the borders. There are visual ways to soften things up so its not as pronounced.