I don't know, the first quote I posted sounded more definite and if he had been involved from the start he would have said 'we' instead of referring to Takara-Tomy as a separate design team when bringing up the prototype.
I prefer to go by the Interview which is a primary source while that statement is secondary. Also it is a bit open to interpretation. He could have meant the beginning of when he was brought into the design team."When Takara came to Shoji with the Masterpiece Starscream project they gave him lots of free range in the design. Shoji was adamant from the beginning that he wanted to design a realistic looking jet, and the robot mode would be secondary."
Furthermore TakaraTOMY's official MP Starscream web site specifically credits Kawamori as the design supervisor (デザイン監修).
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All that is beside the point anyway as the Kawamori 'improvements' were the defining changes to the protoype but as has been mentioned before, the mold changes were not as bad as the actual manufacturing execution of MP-03 because the tail fins and hip kibble can be managed and Takara Tomy did good at sticking to character colors and stronger plastic with later releases of the mold.