Sometimes - to make things easier to line your new objects with your existing map - I scan and print the area I want to change with the area already digitally erased. That way, I can redraw and insure it matches the surrounding area. I then mask out all to just my redrawn object.

Sometimes I print the area, then use tracing paper, something more translucent so you can see the original print beneath, then redraw to lineup exactly.

It depends on which is easier for you. This way you can insure your line widths match up as well.