Kumamoto Japanese (KJ) has a nominative/genitive conversion that is similar but different from the one commonly observed in Modern Standard Japanese (MSJ) in that it can also occur in the root clause. Although many studies have been made about the syntactic nature of the dialectal construction, virtually no investigation has been made so far about whether and how the availability of genitive (and nominative) subjects differ across different word orders and different age groups among the speakers of KJ. Our large-scale online survey involving 400 native KJ speakers aged between their 20s and 60s uncovered three important findings: first, genitive subjects are significantly less acceptable than nominative subjects in both SOV and OSV orders across all age groups; second, there is no significant intergenerational difference in the acceptability of genitive subjects in KJ; third, younger speakers show declining acceptability for nominative subjects in OSV order. We hypothesize that in KJ sentences with a genitive subject serve as presentational sentences, whereas those with a nominative subject are more and more likely to be interpreted as a focal construction. We believe this hypothesis leads to a broader understanding of the nature of the partially overlapping morphological case systems in KJ and MSJ.