Flexi allows you to work with colors in either color space (and more) in the same file. You'll need to select the vector shape that you want to change and open the advanced box on the fill/stroke editor and select the color mode:
If you're wanting to change the color mode of a bitmap element, right click on the picture and select color mode from the menu at the bottom:
Just note that you'll likely be reducing your gamut when you do this.
Good Luck