2011 Volvo 122 Dose Sometimes Replace Fully Usable Spare Parts New Part Number

Not sure what you mean by "fully usable spare parts", but if you mean you try to buy a new part from the dealer and they give you something with a different part number, it's typically because an improvement was made to increase reliability, a change was made to address a common failure, or some other change was made that is significant enough to want to tell it apart from the old version. "Superseded" parts and part numbers are real common.