Probably depends on the color & condition of the hat. I did this with a lighter chestnut-colored brown adult-sized hat for my son. The result was unsatisfactory for anyone but a child, because a discolored ring was left on the brim where the old "foot" had been at the junction of the crown and brim. I had a black vintage hat professionally re-sized for my son though, and it came out just fine. I have had success resizing up. Nutshell, it is better to make a small hat bigger & hide the ring in the hatband than to try to make most hats smaller.
"Faint hat never won fair lady."
"Faint hat never won fair lady."