It doesn’t matter if the dimension starts off as a radius, diameter, or a linear diameter dimension (on a drawing in a model). One can become another quickly in SolidWorks 2009.
Step 1: To change a radius to a diameter, RMB click on the radius dimension. Choose Display Options>Display as a Diameter.
Step 2: To change a diameter dimension into a linear diameter dimension, RMB click on the diameter dimension. Choose Display Options>Display as linear.
Unfortunately, there is no way to shortcut these steps from a radius dimension to a linear diameter dimension. If starting out as a radius, both of these steps will need to be followed in succession to get a linear diameter dimension. Same goes for the reverse.
One word of caution when switching to a linear diameter dimension though; it will often not come in aligned to the Y or X axis, which may render it unuseful for certain circumstances.