Piping modification in a process plant

A new project for changing the desuperheater valve to a new different type requires line size to be changed form 200 NB to 150 NB pipe .And also for this a new change piping modification also need to be done.
What are the points to be considered for this change.
1.Additional line number to incorporate the size change to be made?
2.Welding to a new piping to an old piping -how this is to be analysed?
3.What all to be considered for this design modification.(MOC)

Regards