One who resists change
(Other definitions for diehard that I've seen before include "One who won't accept novelty" , "One to resist change" , "Obstinately conservative" , "One strongly opposed to change" , "traditionalist" .)
(Other definitions for die-hard that I've seen before include "Resolute" , "Reactionary (supporter)" , "Stick-in-the-mud" , "One who won't accept novelty" , "Obstinate or dauntless person" .)