Why not tell us the details about the piping system in question?
The material:
The size:
The flange rating:
The maximum (planned) operating pressure:
Etc.
I have tried to rerate it to new code but calculated hydrostatic pressure is more than actual test pressure for the line. It means hydrotest is required to new test pressure. Perhaps Rehydrotest can be avoided if we use 1966 code. After all piping was designed to that code.
The lines are 2" to 8" in size. Flanges are 150#.Material API5l Gr.B. Pressures vary from 2 barg to 12 barg.