Refreshing existing siding stain that is not peeling?

We bought a "house in the woods" in 2008 with a very dark brown Olympic oil-based semi-transparent stain on cedar siding (found a can in the garage). It's not in bad condition (not peeling or failing, and doesn't get a huge amount of sun) but has faded a bit in places, especially near the bottom where rain water has splashed on it. The previous owner also filled cracks with dark brown caulking which has a a bit of a sheen.

We're in the process of repairing, restoring and repainting wood windows that have suffered badly over the years, and just want to freshen up the siding without making this a major job as well. Can we just clean it and put the same stain right over the top without stripping it? What if we change product? I'd actually prefer to switch to a water-based stain, but the existing Olympic stain is oil-based.

• (1) Can I put a water-based stain right over the Olympic?
  (2) If not, can I just freshen it with the Olympic used before?
  (3) If I can switch to a water-based stain, what would be a good option?
  (4) And if I do switch, might it be a good idea to go for a solid stain to help cover the caulking and hide other imperfections? We would stick to the same dark brown (Oxford Brown)
  (5) What should we clean it with if we don't strip it down to bare wood?

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