Switching to higher res display

I have a T61p. Want to switch between the on-board display (res: 1680x1050) to a 24' Samsung LCD (res: 1920x1200). When switching to the LCD, the resolution does not change to 1920x1200. This doesn't happen even if the -r option is given as an option to the disper command.

Is this because the smaller display can only take 1680x1050 as its highest resolution?

These are the commands I'm trying.
1) Switching to on-board display:
    disper -s
2) Switching to LCD:
     disper -S -r "1920x1200"

Thanks.
-Bhairav