We are in the middle of renovating a kitchen and bathroom. House was
built in 1925, and kitchen pantry was converted to a bathroom I'm
guessing in the 1950's. Current plumbing vent for the bathroom is 2"
copper run on the exterior wall of the house. This vent will be moved
into the wall cavity. (Already have the walls ripped out and holes in
top plates drilled.) Hired plumber will be doing most of the work.

I'd like to reuse the current copper vent pipe inside the wall. I have
no sound reasons for wanting to reuse the pipe other than I just like
the idea of reusing old materials rather than always buying new, even
if it adds to the cost of the job. We'll be asking the plumber about
this, and I anticipate that his answer will be that it is way easier
and cheaper to just run new PVC inside the wall than trying to clean up
the pipe enough to sweat new couplings on as sections of the pipe are
stuffed up the wall cavity.

Also, current waste pipe for the toilet is 3" (I think) copper. This
had to be cut in one place as part of the renovation, so it would only
take one coupling to reconnect the existing waste line to the new
stuff.

So my question is, how hard is it to sweat a 2"+ diameter copper pipe?
I've done plenty of 1/2" and 3/4", so I know how easy it is to do that,
but does it become exponentially more difficult to sweat larger
diameters? Is the plumber going to run screaming from our house if we
ask him to do this? I assume he will want to just rip everything out
and re-run PVC, so should we let him do this?

Thanks for any insight anyone can give me.

Ken