I am removing a piece of galvanized drain while in the process of replacing the vanity.The galvanized is connected to the abs drain pipe entering the waste stack by a rubber connector. In order to reach the clamp screws, I’ve had to remove drywall from a basement wall, but at least I can now reach it. Because the new vanity is taller, and since I’m removing the length of Galvanized pipe in the wall, I’m wondering if I should raise the ptrap. Other posts have made it sound arbitrary depending on the fixture height.
raising the trap will give you more cupboard space, so if it's not too much effort it's a good idea.
You cannot just arbitrarily raise a trap to whatever height you want, it depends on the configuration of the venting system.
For example, if your lav sink drains into a horizontal wet vent type the trap weir must be below the vent connection height.