If I were you, I would start with the 8,5 m2 sail and keep the GO.
The GO boards from 2000 to 2002 are still competitive in speed and early planning.
With your weight, 8-9 m2 sails on the GO give you very good light wind performance. Buy a good fin for the 8,5 m2.
Maybe a controversial statement, but it is true :-)