Moving the batteries would not seem to me to do anything especially useful for you. The 6V batteries should outlast the 12V ones with basic care; they're a good bit more robust, being deep discharge batteries rather than "dual purpose" batteries that are closer to starting batteries than anything. Do check the electrolyte levels from time to time and add distilled water as needed.
Periodical brief higher voltage bursts are good for the batteries in keeping the electrolyte from stratifying and maybe doing a teensy little bit towards keeping the cells equalized. It's certainly not a grave concern. If they are being held there all the time that's a different matter, of course.
