dpb -- I'm with you on doing it right the first time around. I'm all
about that. The problem is that this is a cookie-cutter, factory
assembled staircase rather than a custom built one. Which means that
there's tons of glue on the other side. While not impossible to do,
it increases the scope dramatically.

Wayne -- you've hit the nail on the head! Thank you.

The new tread nosing has sufficient overhang incorporated into it; so
I'd rather not extend existing overhang for multiple reasons as stated
by others above. This means I have to eliminate existing overhang,
which translates to trimming existing tread nosing flush with riser.
And this is where my concern lies.

If you look at this picture -- http://www.masterpiecestaircase.com/...ringer_img.jpg
-- the left (routed) stringer is what I have. If I trim tread flush
with that type of stringer, I cannot remove whatever is left of tread
from stringer groove; mainly because the tread is press fitted and
there's a shim on the underside pushing it against the front of the
groove.