I am adding a bathroom in my attic. Currently no plumbing. I have consulted with licensed plumber who suggested I drop a 3 in. line down the wall over an existing downstairs bathroom. Currently there exists a 2 in. vent for downstairs toilet. He suggested replacing 2 in. vent with the 3 in. line. It would then be used as a wet vent (drain for upstairs, vent of downstairs toilet). I have cut in to the wall behind the downstairs toilet and it appears the 2 in. vent goes below the concrete slab foundation where it ties into the main drain. My question is how deep will I have to jackhammer the foundation to expose pipes enough to tie in the 3 in. wet vent, and how will I tie it in? Can plumbers cut out a 2 in. vent, then enlarge the opening to fit a 3 in. pipe? Or, will the entire intersection of piping have to be cut out and replaced (meaning a considerable more material removal and more work).

Thanks.