Continuance. Except as otherwise provided in this section, the lawful use of any sign existing at the date of the adoption of this chapter, which is not a portable or temporary sign as defined in subsection 22-610.1 above, may be continued although such sign does not conform to this chapter, provided that no nonconforming signs shall be enlarged, extended, increased or changed in material, character, location, illumination or message. Signs of a portable or temporary nature existing as of the date of the adoption of this chapter shall be subject to the provisions of this chapter and as such same shall be removed upon this chapter becoming law.