Here is my kitchen sink drain configuration:
(The horizontal run after the trap is actually pitched correctly, not as drawn.) The portions inside the wall are galvanized steel pipe. Under the sink, the galvanized pipe transitions to white plastic trap and tailpiece. Under the floor, the steel pipe is coupled to cellular ABS drain pipe.
While I am replacing kitchen cabinets, it would be convenient to move this drain line to the floor. Is it kosher to cap the existing line inside the wall, then join a new vertical leg to the existing horizontal run under the floor, as drawn below?