Blake,

Assuming the 84 is set up the same as an 87, yes you'll lose some
coolant.  Depending on how the van is parked, anywhere from a pint to a
gallon or so.  If you've got something that you can catch it with, you
can just put it back in when you're done.  Otherwise, have some on
hand.  You'll want to make sure you bleed the system as well when you're
done, although chances of air bubbles are slight if you only lose a
pint.  I lost about a gallon when I switched mine and did get air in the
system so I knew I had to bleed it.  Make sure that the van is
completely cool when you switch the sensor as some coolant will get on
your hand as you pull the old one out.

You should use a new o-ring as well.

Tonya
87 Vanagon GL Wolfs
74 S. Beetle