3drename (and most other programs) use the Unix 'stat' facility to check if a file with a given name exists, and don't check filename case at all.
It is my guess that you are using a Macintosh, where the filesystem keeps track of the case of names, but you can't have filenames that only differ in case -- that is, you can have a file named 'Abc' but not then have (in the same directory) a file named 'aBC'. This behavior is unlike other Unix systems, but Steve Jobs has his own way of doing things. The ways of the Steve are passing strange, indeed.