If you don't want to increase the output, just use the same coils and either mount the magnets at 12" and keep the coils the same or mount the magnets at 13" and move the coils out to the new radius. You may have to increase the stator moulding diameter to bring the fixing points clear of the discs.

Regarding the coil centres and magnet spacing, for a single turn coil you can put the turn in exactly the right place. You can add more turns on top of each other and still keep them in the right place, but to use more of the available you are forced to put turns side by side, now you can no longer have the ideal condition for maximum voltage from all the turns and those turns that are longer or shorter than the pole pitch will have less voltage induced in them.  It may seem logical to keep the centre of the coil sides on the pole pitch, but in practice it is better to keep the outsides on the pole pitch and then the centres of the coils come inside the ideal. the reason is that long turns have a higher proportion of inactive copper at the inside and outside and give an increase in resistance that out weighs any advantage from the gain in voltage.

With single phase windings the spacing is such that you normally end up with the centre of the coil legs on the pole pitch, but it would possibly be better to leave alternate coils out and wind the remainder with a shorter pitch.

With 3 or 5 phase i.e 9 or 10 coils for 12 magnets you end up with a compromise that is not perfect from the voltage consideration alone, but will give more output power as the winding resistance is less.  Voltage alone is not the thing to aim for, that is why it is difficult to predict what will happen if you change something on a winding.