Can I enter a cost change I found out about late?

Yes, by setting the effective-from date to when the change really started. The days from that date forward are re-priced with the corrected rule, and everything earlier keeps the value that was effective then.

This is the one intended way a closed day changes, which is why it is worth being deliberate about. A month that moved should always have a reason attached, and a backdated cost is the reason that ought to be available.