Soft product is named as such because it's not specific to the physical characteristics of an aircraft; it refers to the service provided (e.g., beverages, meals, hot towels, flight attendant service, etc.).

I can't speak to differences in engines across Delta's fleet of 763ERs but you can track which aircraft in the fleet have been retrofitted with new cabins here:
The Official Delta 767-300ER Interior Mod Tracking Thread