Fact: Any "licensed tile contractor" would know that grout will not bond to grout without the use of an additive such as acrylic latex admixture.
Fact: Modified grout installed over a properly prepared grout will bond. However, subtle differences in color will be noted between the two and these changes will become more profound over time.
Fact: Rich want's his job done right. Listen to Steve.
Fact: Your website has a big button that says "Ask the Tileman". Instead of trying to convince everyone to go to your site and be disappointed why don't you hang around and ACTUALLY answer some questions.
If you need additional information on ceramic tile here is a real website: