Hello. I am doing a bathroom in westchester ny and I would like to know if it is OK to replace the vent lines and drains that are 1 1/2 in copper to1 1/2 in pvc or does it have to be 2 inc pvc

Just so we ares your you are talking about your plumbing drain waste and vent stack pipes, right? Your sink can be 1 1/4" and a tub 1 1/2" but a shower requires a 2" Does this answer what you need?

Usually, toilet requires 2" vent. Tub, shower and sinks can have 1 1/2" vents. Vents can be combined below the ceiling into 2" vent and continue through the roof with one 2" vent.

Drains: Tub and shower requires 2" and 2' trap, sink 1 1/2" drain and 1 1/4" trap.

Sure, it is just fine to replace copper drains / vents with PVC or ABS. Make sure you use correct Fernco shielded couplings when connecting copper to plastic.