Change drive case-sensitivity without reformatting

I format my Time Machine drives as APFS case-insensitive (just APFS). When the drive is mounted and used by Time Machine, it changes this to case-sensitive. If that is what you are seeing, then it is normal.

If this is not a Time Machine drive, then you will need to reformat it to case-insensitive which will be data destructive. For APFS, just that name implies case-insensitive.

If it is a data only drive, then you can create a new APFS volume with the default case-insensitive option alongside your existing volume. This can allow you to copy the files directly from the old volume to the new volume. Once you are satisfied everything is Ok on the new APFS volume, you can easily delete the old Case-Sensitive APFS volume. See the following Apple article for details:

Add, delete, or erase APFS volumes in Disk Utility on Mac - Apple Support

FYI, you really should have a good backup of all the data on all of your external media (including the cloud) if the data is important & unique, otherwise you will lose it at some point.