I want to regrout my shower floor with another color grout. Can I go over the existing grout? Or do I need to grind out some of the existing grout lines. Also will new grout adhere to old?

I want to regrout my shower floor with another color grout. Can i go over the existing grout? or do i need to grind out some of the existing grout lines. Also will new grout adhere to old?
Your going to have to grind some out. Dremel makes a nice attachment for their moto-tool to do so.

They do make stains for grout you may try