Humm, this could be problematic. You might get an exemption, but a shower drain, in contrast to a tub drain, is supposed to be 2", not 1.5".
What you might want to look into is maybe using a trench drain (not sure that is the right name)...basically, instead of sloping the entire floor towards the middle, you slope it toward one end - the end where the existing drain is now. The floor would fall off to a trench the width of the shower. This technique is used in gang showers, like in a shower room at a school where it would be tough to put multiple individual drains.
Not sure of the details of how to do this, but worth looking into in your situation.
Where I live, the city codes do not allow anyone but a licensed plumber to do plumbing in a multi-family dwelling...you may want to check.