The provisions of this subchapter shall not be construed as affecting any permit or valid existing right-of-way granted prior to June 10, 1920, or as confirming or otherwise affecting any claim, or as affecting any authority heretofore given pursuant to law, but any person, association, corporation, State, or municipality holding or possessing such permit, right-of-way or authority may apply for a license under this chapter, and upon such application the Commission may issue to any such applicant a license in accordance with the provisions of this subchapter and in such case the provisions of this chapter shall apply to such applicant as a licensee under this chapter: Provided, That when application is made for a license under this section for a project or projects already constructed the fair value of said project or projects determined as provided in this section, shall for the purposes of this subchapter and of said license be deemed to be the amount to be allowed as the net investment of the applicant in such project or projects as of the date of such license, or as of the date of such determination, if license has not been issued. Such fair value shall be determined by the Commission after notice and opportunity for hearing.