I would suspect they need to use the new codes.
However, the refinery would be the owner and they can do what they want, baring any prohibition from government regulation.
Using horse sense, I'd say the following:
From a strength point of view, the 1930 stairs are plenty OK. Along 80 years they've suported millions of people going up and down, many of them carrying weights, without breaking down.
The problem are new safety regulations, the most important being the length of handrails: at the bottom of first step and the end of last step they should protrude farther than the original stairs.
If I were the onwer, I'd decide to copy the existing stairs, as proposed by the contractor, but making the handrails of the length required by the new safety regulations.
Giovanni S. Crisi
Sao Paulo - Brazil