As I understand it, the places which fit the lenses carry (or make) a range of sizes of lenses and trim/fit to the frames. So, unless the 'old' frames are so special there isn't something which can be trimmed etc, it sounds like a fob off.
Some years back, for some reason I didn't have a spare pair and broke my only pair- which were all but new. A visit to optician and they spotted a fault in the frame* - I confess I had assumed it was my fault. The style was LARGE lenses. They managed to fit the lenses in some frames in the shop - how I don't know- and ordered me a new complete pair plus a spare pair, free for the inconvenience!
Senior Management always has two pairs (as I do now). She often likes to keep one of the old frames- not on cost grounds, the second pair is generally 'free' (supposedly)- she just seems to like the style. Likewise, once I've found a style I like, I tend to be reluctant to change it and prefer to keep the old frames. I've never had a problem.
Try Specsavers!
- they were supposedly unbreakable apparently!