RE: Start centerboard?
1. Longer stern fins can help (all of Starts came with a 40 cm deep Tuttle fin for the stern).
2. I think there isn't any upgrade for the retractable "clipperbox" daggerboard for the center which is on the 2004 and later models.
3. The fixed centerboards of the 2001 and 2002 Start used a short Tuttle fin box and can be upgraded with longer fins as centerboards. (I've improved my upwind ability on my 2001 Start using a 54 cm fin as the centerboard.)
4. The 2003 Start used deep Tuttle boxes at the center and the stern and advertized being able to interchange the front and stern fins.
5. I believe more of the problem is a technique issue rather than an equipment issue. I would suggest posting a question about likely technique problems on the Windsurfing school forum for Rger may be much more helpful than equipment additions. Weighting the stern, keeping the sail leaned somewhat away from the wind, and slightly tipping the windward rail all help going upwind in light wind.