Once you're on 15, there's no way back, unfortunately. The EU is trying to frame laws to make it illegal for companies not to allow software downgrades. If they can really put those kind of laws into force, there is some hope it will become possible to downgrade your device to any previous version of ios - that will be the best thing to happen to the jailbreaking community in a long time. Of course we know Apple wants to keep us locked out of our own devices, so I'm not 100% optimistic on this. Even if they make it possible in the EU to comply with new laws, they will probably fight tooth and nail to keep everyone else restricted. Keep your fingers crossed.