According to Porsches 2001 prospective buyers 80 page brochure, yes you can shift when in D. "A new Tiptronic S feature allows you to instantly take control of the shifting even when the gearshift lever is in automatic. At the press of a rocker switch, the system reverts to manual mode for at least eight seconds, allowing your fingertips to shift gears on the fly, and devour unsuspecting hairpins at will."
Wow my first post and it's an answer/observation and not a newbie question. I've had my Speed Yellow 2001 S w/ Tip about 2 1/2 weeks and love it. Just wish it wasn't so hot here in AZ. Can't drive it during the day it's just too hot. 106.
Chris