The owner of any sign which is otherwise allowed by this subchapter may substitute non-commercial copy in lieu of any other commercial or non-commercial copy. This substitution of copy may be made without any additional approval or permitting. The purpose of this provision is to prevent any inadvertent favoring of commercial speech over non-commercial speech, or favoring of any particular non-commercial message over any other non-commercial message. This provision prevails over any more specific provision to the contrary.