You don't have to remove the dash or any part of it to change out the light switch. The correct one for the RS also has two connectons for the vacuum lines that apply/remove vacuum to the top of the relay valve (mounted to the vacuum tank on LH inner fender at the front).
This has also been discussed at the following link: http://www.camaros.org/forum/index.php?topic=656.msg3653#msg3653
As William stated, and also discussed in the link, there is a plunger that has to be depressed to diengage the light switch knob. Even after you get the plunger depressed, the knob probably will not just come out. You have to be patient and wiggle it and encourage it - especially if it has been there a long time. Once you get it out, remove the nut on the dash side of the switch and remove it.
Attached pic is the original switch I removed from my 69 RS last year. You are looking at the bottom of the switch, as you'd see it if you looked up from under the dash. First remove both vacuum lines, noting which line goes to the #2 connection and which line goes to the #3 connection, depress and hold the plunger, and "wiggle" the knob out. A little lubricating spray down the shaft of the knob won't hurt either.
Be patient - it probably will not just "fall out in your hands".
Keep us posted and good luck.