Could be done with your FL die but due to the fairly large step involved, it would be best to do it in steps. Obviously, a bushing die would be easiest for necking down but there are other ways.

Also, brass with thin out while necking up and thicken when necking down. Necking up would be best accomplished with a tapered expander and plenty of case lube.

Necking down can also be accomplished with other dies you may already have. Just gotta think a bit and experiment. For example, you might be able to use a seating die for a 243 or 260 with the seating plug removed to make a first step from 308. Get the idea?