Just offering an alternative. There are boots like the salomon ghost 100 which you can adjust the flex of on the hill. There is a bolt in the back which if its there the flex is 120 and if you take it out its 100. All it takes is a 5mm allen key. So if you want powder in the morning, keep the bolts in, then if you move to the park, just take the bolts out and put them in your pocket. Much easier to carry an allen key then a spare set of tounges.
The ghost also has a 98mm last but also has custom shell to aid fitting. Also its not the only boot you can make adjustments on the fly with. Most boots offer some adjustment and usally its much qicker and easier then swapping tounges.