My suggestion:
Copy the shapes of the brown edges on the original texture... Rescale it to the new size, and draw, on a new layer, the same shapes, but don't follow the exact pixel corners. Follow the implied edges freehand. (if something looks like it should be round, make it round.) But don't use any blur.
Did that make sense? I really think the only way to get a flowing texture is to copy the shapes they originally used. As for corners... well... they didn't bother to make them flow, so you'll likely have to deal with whatever happens there...(by that I mean, "Put up with") OR add geometry to the corners, that you have textured yourself. (But as I don't think map geometry is currently moddable....)
If you could SOMEHOW manage to get the game to read some other file format for 3D map mesh... you could use a new model entirely... but that sounds even more difficult and pointless...(I think Aali is probably the only one who could even attempt it and I don't think he knows much to do with 3D... could be wrong... but I don't think it's his specialty.)