So I am looking to get new boots. As I don't always have access to a certified boot fitter, a few weeks ago we were near one so I decided to stop in and try a couple of boots on. As I stated to the fitter that in my current shell I can lift my heel quite a ways he suggested I try a 26.5 shell. (Currently in a 27.5) The shell I wanted to try was a Lange blaster Pro. He placed an insert in the 26.5 shell in what he stated was an attempt to gain room. I suggested that we try the insert in the 27.5 shell for comparison purposes and he stated not to bother as I would just swim in it with it in. Can someone explain to me how this works? How does adding an insert gain you room? While my ankle was certainly locked in the 26.5 shell and I didn't feel any discomfort at the time in store conditions, I was concerned about spending a full day in the boots in freezing temps. Should I be concerned? Can any boot be modified in this manner?