HELP !!!

I just bought an apartment in a new building.....Apt. has two bathrooms with bathtubs in each one.

I hired a contractor to do work in the apt.

First on my list , is to remove tub in master bath and put in a stall shower.....

Contractor just called.....Plumber says can't do because code for a tub is 1 1/2" drain and code for shower is 2".

Builder is trying to find out size of existing drain...It is a major job,I'm told to change the drain....

Any suggestions ????