If the petroleum product you store on site changes from diesel to gasoline, you have made a material change in your operation. You have gone from a combustible product to a flammable product. You must revise your plan. If you change your grade of gasoline from one octane to another, that would not be considered to be a material change.
Another point to remember about SPCC is that it refers to the amount of petroleum product you have onsite at any given time. So you are a marketer of gasoline and you purchase 5500 gallons at one time and then you deliver smaller quantities to your customers over the year until you have 500 gallons left onsite. You are still required to have a SPCC plan since you have more than 1320 gallons during the year.
We discuss SPCC requirements at our 2-day environmental seminars.