Check first with your optician if the old pair will take the new lenses, giving a spare pair, but if you want to clear space, you can resell on a site such as mybinocle.com. Prices depend on the designer, with Prada, for example selling from €25-€150.
Many opticians will take old pairs and many of these are passed on to Lunettes sans Frontières in Haut-Rhin, Alsace, or Médico Lions Clubs de France in Le Havre, Seine-Maritime. Others, such as the Krys group, have their own recycling operations with Krys recycling 160,000 pairs in 2017.