A very useful function in Emacs is
describe-key that will tell you what function is being called by which keys.
So, it seems that in AucTEX, C-c C-f C-i calls
TeX-font and then passes C-i as argument.
So a simple solution would be to bind a key to TeX-font and keep the C-i (and others) to get the font you want. You can customize the list of possible fonts and secondary key shortcuts with
You would thus have a simple:
(define-key LaTeX-mode-map (kbd "C-S-i") 'TeX-font)
(with whatever key you want).
If you really want to only rebind the italics and nothing else, you can always do:
(define-key LaTeX-mode-map (kbd "C-S-i") '(lambda () (interactive)(TeX-font nil ?\C-i)))
no need to redefine the whole behaviour of TeX-font.