In my basement I have a standard washer hookup. Hot and cold supply
lines lead to a single valve with a vertical lever that, when down,
allows hot to go to the hot washer hose and cold to the cold washer
hose.

The single valve with the vertical lever (I do not know what it is
called) is failing. Water is slowly dripping from the interior of the
lever area, which looks rather corroded. The value is probably 40-50
years old.

I want to replace the valve, and it looks like there are two screws
facing the front that hold it on the hot and cold supply lines. I
unloosed one of the screws and received a nice spray. This is a good
sign (even though I got a bit wet) in terms of what the screws do.

On Monday I am going to go to a plumbing supply place and get a new
washer valve (hopefully they still make the same size valves...)
Assuming I do this, I see one problem. For the hot supply line, there
are no shutoff valves between the washer valve and the hot water
heater. For the cold supply line, the only valve between the washer
valve and the supply line for my house is a shutoff that will shut the
water off to the entire house.

So, I anticipate that I am going to have to turn off my main house
shutoff valve before I remove the washer valve. Is this a risky
thing? What is the main house shutoff valve fails?

Will I have to drain a ton of water after turning off my main house
shutoff valve? I presume that any water above the washer valve will
want to come down and out where I am working after I remove the washer
valve. Should I turn off the house shutoff and then, say, open an
upstairs sink fixture to get rid of any pressure upstairs before I
mess with the basement washer valve?

Thanks,

mh