I am currently finishing the basement in my home built about 2 years ago. The people that did the plumbing and heating didn’t want to spend the time to keep all of the plumbing and ductwork up in the joists so I’m having to move a bunch of it. I actually have 2 questions:

1. I have an A/C lineset that is hung below the joists that I’d like to move up into the floor. How difficult is it to remove that lineset and move it? It’s running perpendicular to the joists so I know I’ll have to drill them, but I’m concerned with the actually disconnection and reconnection at the condenser and coil. I also need to move the gas line in the same manner. It’s a flexible (wardflex or similar) gas line. My concern with it is not causing a fire with sparks, etc from a wrench and how to deal with the bleed off the gas after I disconnect it.
2. I want to install a half bath in the basement. I have planned to locate it directly adjacent to an existing DWV stack that extends under the slab and I believe is connected to several other under slab waste lines. Our sewer tap is below that level so I know that lift station is not an issue. I’m planning to sawcut and remove the concrete slab around the underslab waste line and tie into it in the necessary location. My concern is that the line may not be low enough to get the necessary drop from the level of the flange to the tee. What drop is necessary? I also plan to tie the lav drain into the side of the same stack above the floor, but I don’t know what the venting issues are. Can I just run a 1-1/2” drain into the stack and be done?

By A/C "lineset" are you referring to the A/C condensate line? If it is then you will have to give it 1/4" slope to the foot when you rerun it. You shouldn't have to disconnect at the condensate pan, two coupings should do the trick. Shut the gas off and bleed the line down in a ventilated room before you disconnect anything.

The slope is the same for drainage as it is for the condensate line. 1/4" to the foot, although you may cut back to 1/8th" per foot if elevation's tight.
The lavatory may discharge into the stack only if you vent it. This can be a vent that runs to the roof or revents back into a existing dry vent in the attic. If local codes permit you might consider adding a AAV, (Air Admittance Vent) just after the trap and that would solve your vent problem. Good luck, Tom