No, even when modified the plane keeps its original designation, but often letters are added.

Take for instance KLM’s original short upper fuselage B747-206.
They had their upper deck stretched changing them to B747-206SUD.
After that they where modernised making them B747-206SUD/M.

Same goes for many B737-200. Many where modernised after which A. or Adv. was added behind the original code.