move to case-sensitive APFS?

I can’t say for sure that it’s impossible but am not aware of any way to convert it without formatting.

It was possible to convert volumes from HFS+ to APFS “in place”, so I guess it is not impossible to implement what you are asking.

I am just not aware of such a tool being available. One would have to implement this at a very low level, probably only Apple has the resources and know-how to do it.

I don’t, however, see any pressing reason to do it, as very few people even know about case sensitive APFS, or would have a reason to use it. It’s unlike the conversion from HFS+, where Apple needed everyone to transition to the new format as painlessly as possible.

Note: I am not in any way arguing against it, you probably have a good reason to want this.

I don’t think you can do that.