I was going to post that I didn't think they'd let you swap, since they've had them made up specifically for you and can't resell them. But their website says "We want you to be completely happy with your purchase at Specsavers Opticians. If you have any concerns within six months of the date of your purchase we will put it right. No quibble, no fuss." so you might get lucky if you go in and ask. No harm in trying!