Uncapitalize (verb)
To convert the first letter (or more) of (something) from uppercase to lowercase; to make uncapitalized.
Decapitalize (verb)
To uncapitalize convert the first letter (or more) of (something) from uppercase to lowercase.
Decapitalize (verb)
To remove (something’s) status as a capital crime; to cause (something) to no longer be a capital crime.
Wiktionary