At least with the old SAT, the test given internationally in May were generally the same as the test given in the U.S. in May. The reason was that May was the only international QAS date, and College Board thought it more important to release only one Saturday test (rather than two if it used a separate test outside of North America) than to prevent cheating.
Of course, there is no telling whether that will continue to be the case with the new SAT. The College Board would open itself up to a lot of criticism if it reused the March test just two months later.
Regardless, it is difficult to see what connection any such concerns would have with adult test takers. What cheating scandals in the recent past have been connected to adults at test centers? And if adults at test centers present a security risk, why not simply ban them? Instead, College Board has reassigned them to the May test date. If adult test takers pose a risk, why are they a risk in March and not in May?
Most importantly–even if we grant that there is a potential problem that affects the March date exclusively–why was College Board unable to anticipate it and deal with it more than five days in advance? In a way that would cause minimal disruption? This is basically their only job, and yet this latest problem is hardly an anomaly.