I need to replace my prescription sunglasses and I'm pretty sure I'm going back to Costas. I checked with my eye doc and their frames are pretty much retail prices. I did a quick search and Sierra came up with the frames I want for about half price. The only thing I'm not sure about is the lenses that come with them are the 400G, not the 580P that I want. Are the specs for the P's the same as the G's in the way of size/thickness (is this an easy trade out)? Anyone ever ordered from Sierra?