According to French technology site PCInpact (via Electronista) multiple readers of the site have been contacted by Orange regarding upgrading to future iPhone models.
Apparently two solutions are on the table, the first entails a direct swap. A trade-in for the current model would almost fully subsidise the new handset leaving just €50 to be paid off.
Alternatively existing subscribers could obtain the next-generation iPhone at a "generous discount", currently the exact amount is unknown.