There should be no need to replace the spring. But, you are right about it changing the opening pressure over time.
We have always rebuilt our own injectors by installing new nozzles and setting up the opening pressures with larger shims. Most injectors loose 2-300psi opening pressure over 100k miles.
As long as the shim size is available it is not necessary to replace the springs.
Owner 24 bay BSC
Bosch Master, ASE master L1
26 years MB technician