Continuing from "gray" ("grey" wouldn't have worked anyway, even if it had been first; it drops the "a" and adds an "r" and a "y"):
angry
I'm so sorry! I mistakenly thought I was in the "change a letter in the word above" thread. Please forgive me!
"gauntry" isn't in m.w, so, continuing from "gantry":
Langtry
There is at least one word that can follow MATURENESS.
