Why do I need to reformat to make Case-Sensitive?

The ONLY difference between a volume/partition that is formatted as CS or Non-CS is a flag in the directory. Since a Non-CS partition can NOT have file names that would be duplicates if rendered as all lower case, all that SHOULD be needed to go from Non-CS to CS is to flip the flag and update an in-memory tables that show the CS/Non-CS status. An attempt to convert a CS partition to Non-CS is harder and would actually need for the partition to be unmounted (to prevent adds/deletes) while the directory is scanned for duplicate Non-CS file names before the flag is reset or an error message was issued.