I'm not an optometrist but I have one anecdotal data point for you.
When I first got prescription sunglasses, they matched my regular prescription. A couple years later my 'scrip changed, but I was too cheap to buy new sunglasses, so kept the old sunglasses and just updated the regular glasses. I don't remember the difference in numbers, but I found that when I switched between regular and sunglasses, it took me a few seconds to adjust, but it wasn't a big deal. I think my regular 'scrip changed twice while I had the same sunglasses, and it worked out okay.
Eventually I got tired of carrying around sunglasses, so I now have Transition lenses.