If there is any question about getting a piano through a door or
multiple doorways, I found the prudent action is to make a cardboard
template of the side of the piano, to see which way it might best go
around a doorway.  It's very easy to play with the template and try
different approaches than with the real thing.

I don't speak from a vast amount of experience, but I have moved
one piano, to about 1/2-dozen residences, and through many more
doorways.  I did one move where it was determined that the piano
had to be on one side through a hall door, down the hall, let down,
upended onto the other side to get through another door.  All this
was figured out before hand with a template -- so much easier.

This is something you don't want to try to figure out as you move
the piano.  Make a template, test it out, and you know exactly the
easiest way to do the move, beforehand, with no nasty surprises.

As to dismantling a piano to get through a small doorway, rather
than break down the piano, and have to reassemble it, with all the
attendant possibilities of screwing up the job, I would be looking
into remodeling the door opening.  32" doorways are pretty much
standard here, with some as little as 28" for a pantry or some such.

If a doorway had been altered to be less than 32", it's not going to
be convenient for moving other pieces besides pianos, and should be
corrected anyway, if at all feasible.  It has got to be easier and
safer to remodel a doorway than to take a chance on reassembling
a piano incorrectly.

If you live in an area like San Francisco, where there are many
3-story Victorian or Edwardian 6-unit apartment buildings, you employ
a sign moving company with a crane.  Remove the middle bay window
sashes and bring your pianos and/or organs in through the window
opening, bypassing the stairway and doorways.  So far I'm up to five
of those moves, in and out, with one piano, one full AGO size church
organ, and one three-manual theatre organ.

Tom Lear
San Francisco, CA