Seat upgrade - advice needed

I'm wanting to replace the cloth seats in my A3 2.0 tdi sport quattro 5 door with leather if possible. Straight swap options seem to be S line leather interior or S3 leather interior.

I've seen a few threads about fitting other seats such as from the A5, can anyone advise whether this is an easy conversion? Are the rear seats able to be fitted with mods or will i need to fit A3 leather rear seats? Do any other front seats such as A6 ones fit as they all look very similar in size.

Any advice welcome to avoid potentially costly errors, if you have any for sale, I would be interested.

Thanks