The Paris Opera, like many other opera and concert venues, doesn't accept returns. So if you find you can't use the ticket you bought, then tough luck. Unless you want to take your chances on Ebay, or standing outside on the night - in which case you may well end up receiving less than face value from a tout - who then sells your ticket on at a gain, the bastard.
In an effort to reduce this kind of black market profiteering, the Opera is introducing a new user-to-user ticket resale service on its website from 9 February. Exchanges between users will be supervised by a specialist service called zePass. Resales may not exceed the face value, online payments are secure and successful completion of the exchange will be guaranteed by the platform. The risk of a no-sale remains with the seller - which is no doubt why this solution was more attractive to the Opera than simply changing their policy and accepting returns.
But here's the especially good bit for English persons. A bonus feature is that users will be able to sign up to receive alerts when tickets go on sale for a particular event - useful for those of us who live too far away to chance it on the night for a sold-out show.