No way to downgrade you M8 to work like your S6 and no way to take over system settings either, use app configuration can be moved over using titanium backup, but both devices need to be rooted which would result in your s6 being sans warranty.
Also, just FYI, a sim stores nothing but network data, no messages, contacts, app configuration of any kind, it does not even store the Internet connection settings.
Within 2 to 5 years there will be no such thing as a sim card, Apple has already begun working on simless, the next device us said to be using an apple programmable sim which the networks will have to use, the following device will be truly simless and network setting will be user download able to the device. Changing networks will take as long as changing your underwear.