So I'm watching the DVD version...and, in addition to changing Hiroki's appearance, they also changed his name to Takuya.
His Gureggru got a name change, too.
Best I can figure...Gurekko.
Before, its name was Gurekichi, which is awfully close to Pyonkichi, the name of the frog in Dokonjou Gaeru. So, it got changed.
Luckily for the producers of the show, the only people who say "Hiroki" or "Gurekichi" are Joi, Maxi, and Hiroki/Takuya. So only three voice actors had to be called in to rerecord their lines.
The only other change I noticed was one of Hiroki's/Takuya's lines. Right after he gets up out of his throne to battle Takeshi, Hiroki says 根性、根性、ど根性だ! (konjou, konjou, dokonjou!, or "We've got guts! We've got guts! We're gutsy!"). That third part there is taken directly from the title of the anime Hiroki's modeled after, Dokonjou Gaeru, so the producers changed it to 燃えろ、燃えろ、燃えれ! ("We're on fire! We're on fire! Our fire's really burning!") to be safe.