The election required by this subsection shall be made for a taxpayer's first taxable year ending on or after January 1, 1987, in such manner as may be specified by the commissioner. If a return for such a taxable year has been filed without such an election prior to or within 90 days after March 11, 1987, the taxpayer may file an amended return containing such an election.
OCGA § 48-7-39