Is there a way to remove the 8.3 filename limitation on the SD.h library?
The 8.3 "limit" is not a "limit" that can be removed. It is how FAT filesystems work.  Longer filenames are a "fudge" extension overlaid on top of that fileststem structure. To be able to use them you would need to implement that extension.

Simpler to switch to a more modern implementation.  FatFS is your friend:


LFN support, including UTF
Faster device access
More efficient
Can handle more devices than just SD cards
Can handle multiple devices at once


There is an implementation here though I don't know how good it is (I use it on chipKIT where it is bundled by default).

If you can't get FatFs then basically anything is better than the SD library to be brutally honest.