In the trial on a petition authorized in subsection (a) of Code Section 19-6-19, the merits of whether a party, a child or children, or both, are entitled to alimony and support are not an issue. The only issue is whether there has been such a substantial change in the income and financial status of either former spouse or in the needs of the child or children, in cases of permanent alimony for the support of a child or children, or in the income and financial status of either former spouse, in cases of permanent alimony for the support of a former spouse, as to warrant either a downward or upward revision or modification of the permanent alimony judgment.