Hi!
I have measured the voltage for contrast: it is always near ground (typical value is 0.35V). You can do the electronic regulator and change the volatage, but I am not quite sure if using the transistors fits your requirments. I have saw solution like yours using simply d/a converter made of resistors.
About changing leds: if you have enough money to buy a new one lcd... Try it... I haven't seen you lcd - maybe it is possible. But you can also buy lcd display with blue, yellow or green background.

Regards,
Jacek