Flash has a nifty little button that saves you a few clicks by letting you switch out your selected stroke and fill colors so that the fill color becomes the stroke color, and vice versa. Easier than manually selecting each one again and trying to make sure you perfectly match what you had before, right?
Just look right below the line and fill color pickers at the bottom of your main left-hand toolbar; you'll see a little button with a black square, a white square, and a two-headed arrow pointing to each.

Click that to flip the colors around.