Do I have to change drain in order to put tub in

Hi BB...

Technically, a tub drain is only required to have an 1.5" drain and a shower requires a 2" drain so, yes, you need to change that drain over to be up to code.

With that being said, many people will simply cut out and relocate the drain pipe from the tub and increase the 1.5" drain to 2" using an increasing coupling but I can't recommend that as someone may need to snake that drain someday and the snake could get hung up in the 1.5" drain pipe as someone actually thinks it is a 2" drain pipe and that could be a huge issue depending on the bathroom setup.

Tell me, Is the tub on the 1st floor, 2nd floor, or on a concrete slab? How old is the house? Do you have PVC or ABS (white or black plastic pipe), or cast iron (black metal) and galvanized/copper pipes?

Back to you...

Mark