I agree with Ms. Smith. First try to work with the City. If that doe snot work, hire counsel to help you identify and enforce your rights. Do not delay as the passage of time will negatively affect your rights.
As recommended above you should discuss this with your local planning/development department. If you do not get a satisfactory response there consider hiring a local land-use attorney to review the situation for compliance with local zoning code, and determine if there is a claim that can be brought to enforce any rights you may have.
I recommend that you speak to a local Idaho attorney who has experience with land use and zoning issues. Under certain limited circumstances a non-conforming use can be " grandfathered" in but the pre-existing use cannot be enlarged. The ordinances should define "light industrial". It is possible that the city changed the zoning designation or granted a variance to the grocery warehouse. The city website should provide a link to the local ordinances as well as a zoning map.
This answer is provided as a mere courtesy and is not intended to provide legal advise but rather a general roadmap which may assist in deterring whether or not to seek legal advise from a law office in your locale.. In some cases the answers may not be jurisdiction specific. In no event, is an attorney-client relationship formed by providing answers to posted questions.