Should we sand a fence to restain it with a solid stain?
Many fence treatments are intended for use on rough sawn wood as they soak in better. Assuming your fence is made of sawn timber, you shouldn't sand it. 

Also the pressure treatment doesn't soak in all the way (or at least not at full strength) so you wouldn't want to remove much material. 

A pressure washer or a brush would be a good idea, especially combined with detergent (and then a good rinse, consider what's underneath when you choose detergent). It's possible a pressure washer will remove flakes of old treatment, or even loose bits of wood. This isn't necessarily a bad thing as it will give you a good surface.