In the trial on a petition authorized in subsection (a) of Code Section 19-6-19, the merits of whether a party is entitled to alimony 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, 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.