We're planning on adding a below-grade bathroom. Luckily the plumbing is all accessible.

The P-trap for the upstairs bathtub is attached to the top of a 2inch vertical pipe. The end going thru the ceiling (first floor subfloor) is presumably a vent. The other end drops about 18" and is attached to the outflow pipe.

Can we move the P-trap down about a foot or so in order to add vents for the new basement fixtures above it? Is that a bad idea?