Our company owns an oven that had been commissioned for use in drying volatile paints, classifying it as a Class A oven under NFPA 86. Since discontinuing work on that job, the oven has been left unused for a few years and we would like to convert it for potential use as a curing oven for powder coating. For context, the new process would require increasing the ovens upper limit from ~300F to 500F.
We've contacted the original manufacturer for assistance and they refuse to make any modifications that would divert the use of the unit to anything other than what it was designed for, specifically its previous operation as a Class A oven. That is the sole factor being offered. All components are rated for the conversion.
My question is why the the Class A designation and operation is grounds for considering it unsafe or impossible to modify the oven for reuse in a less volatile process. Does NFPA 86 contain information that pertains to this situation?