I don't think so. I think you need to reformat.
Imagine for a minute that you could turn off case sensitivity without reformatting. Imagine also that you had two files on your Desktop, named:
1) Document1.rtf
2) document1.rtf
On a case-sensitive volume, all is well... but you just can't do that with a case-insensitive volume. Imagine now that you "turned off" case sensitivity on the volume... what happens to those documents? Does one overwrite the other? Are they merged? Does one become a hard link to the other? Are they corrupted? Do they disappear?
I don't think there is any set of rules you could use to solve that dilemma, from a code standpoint (think about how Disk Utility would try and handle that situation -- more than likely, it just can't).