On Thu, 10 Jul 2008 09:23:09 -0700 (PDT),
wrote:

The water has to be drained before the valve has to be
replaced.

A few weeks ago I watched a plumber repair some pipes. He had
difficulty, because of the water remaining and was not able to sweat
the pipes.

His solution:

Using a cordless drill, he drilled a small hole in the bottom of the
pipe, next to the valve. Water drained, and repairs made.

That was the first time I've seen such a trick.