Yes, the nozzle schedule could make the difference, but that is easy to determine in PV Elite;- change the nozzle schedule and run an analysis with the higher DP. Then try to change back the nozzle schedule , add a reo pad and run again the analysis. However, is another story with adding a reo pad for up-rating a vessel...that's the vessel modification!
Is there any corrosion allowance included, have you had a shell thickness monitoring program in place? There are many other factors you have to consider for a re-rate.