Do you need to etch the existing epoxy floor before you apply the new epoxy paint?

Etching only benefits bare concrete floors. If you want to ensure that your new coating will adhere to your old coating, blend enough of the epoxy to cover a single square foot section, let it cure for a week, then check the adhesion by scoring a cross-hatch design in the new coating, put some Duct Tape across the scored area, then quickly pull up the tape. If the epoxy comes up, then you will have to remove the old coating prior to recoating the floor.