You're using gimp? I would suggest pasting the old map, as a layer, on top of the new version at the same size, or so the old islands are just a bit smaller than the new islands. Make a second layer on top of both and re-trace the old island coastlines (without the square pixelized corners, of course). Delete or hide the old map layer. Make a duplicate layer (below the traced layer) of the new map for your working layer and hide the other one so you don't accidentally paint on it. Then rework the water to match the old coastlines. You could try using the smear or smudge tool (or whatever it's called, I haven't used gimp in a while). Or if gimp has a liquify filter like photoshop, that might be easier and you'll have less work to touch up the style design. There may be a simpler way to do it, but I'm not an expert. Maybe you'll receive better advice from someone else.

You're doing a fine job so far! =)