Things to consider when re-coating.
Check the new coatings instructions to determine if they recommend application over an existing coating. Most products will have something about this in their manuals.
Is the existing coating sound?
If the coating is not peeling or lifting, you should be able to apply a new coating over it. Scuff sand the coating with a 60-80 grit sand paper to create a mechanical profile and remove the debris prior to application.
If it is failing- lifting or peeling, remove the coating either by chemically stripping or mechanically grinding it away to the bare concrete.
On a solvent based epoxy, it will try to chemically bond with the existing coating during its initial cure. It basically attacks the coating and "bites" into it forming a chemical as well as mechanical bond. If the existing coating is not chemical resistant enough, it could be "eaten" by the solven based epoxy, essentially turning the floor coating to a soft, gooey mess, making the project more difficult than necessary.
