A process and system for sterilizing packages of thermally-treatable products having different sterilization processing requirements. These packaged products can be different types of canned soups, for example. At least one of the packaged products is preheated a significant, predetermined amount in a preheat vessel in a steam or steam-air mixture to accommodate these different processing requirements. This allows the first and second packages to be processed through the stationary hydrostatic sterilizer, which is generally separate from the preheat vessel, at a common line speed and temperature. Expensive sterilizer changeover delays are thereby reduced significantly or even avoided. This process also allows the soup product to be cold blended, reducing the likelihood of any overcooking in the large cooking kettle and reducing the amount of starch needed.