The only way I can think of to correct this is to go into the past assignments and mark them as not late. In Grades, you can click the icon next to a students grade which will open the sidebar. For anyone marked late, change the status to none. I know it's not ideal to have to make these changes but from that sidebar you can click the left and right arrows next to the Assignment Title and fairly quickly move through the class. (Screenshot below is for a fake student on a fake assignment in a fake class)

Since the Late Policy affects the entire gradebook I think making this change is about the only way to have it only affect submissions going forward.
Rick
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