There's a little-known page on Apple Support's website allowing you to request an exchange for a different-sized light seal. If, like me, you know what different size you want but it's unavailable in-store, this is a great option. I had to run through the questions several times for it to spit out the size that fit me best at the Apple Store, but it's not too hard.
https://secure.store.apple.com/shop/vision/fit/help
https://secure.store.apple.com/shop/vision/fit/help