So the retail on a first chair 6 is a bit cheaper than the first chair 10, could you theoretically just but a first chair 6 and put a 10 or 12 flex tongue on it and basically have a first cchair 10? Or are the shells different..
Yes you can. The first chair series is the same shell, with a different flex tongue for each boot, as you have already deduced. If you buy a first chair 6 and put a 10 flex tongue on it, the only difference will be the color.