I've been on the road a lot for a few days, so I just saw this thread and see that many people have already written what I would have: that even if your prescription changes, you don't need to replace your lenses each time. I am diabetic and have frequent screening for vision and retinopathy as well as checks for cataracts. Regarding changing lenses, I do this only if my optometrist tells me I am getting close to being unable to pass the vision test required to keep my driver's license! I would probably get new lenses if my vision problems made my head ache, if I was squinting a lot, or if I had any other discomfort caused by vision loss. It is nice to know that if my glasses break, my optometrist has a record of my current prescription so that replacement glasses can be made. Also, I very rarely replace my frames. I have a silver-tone frame and a gold-tone frame and I alternate between them. Right now I'm wearing the gold-tone. When I do need new lenses, I'll have them put them in the silver-tone. Then the ones I wear now will become my back-ups in case the newer ones break and I have to wait a few days for new ones.

About the sunglasses: I do what Charmion does, only more so. I have two pairs of those over-the-specs sunglasses designed for people with cataracts. One is small and I put that right over my regular glasses. Then I put the larger sunglasses on top! That blocks out even more sun! And of course I wear wide-brimmed hat.