Kim -

Taking all the strings off at the same time makes it somewhat likely that the relief of tension on the top plate of the fiddle will allow the sound post to drop out of place. The extent of the risk depends very much on the condition of the instrument, but it's real enough that it's advised that strings always be changed one at a time.

Resetting a sound post isn't a technically complicated task, but since you have to do it by working through the F-holes it can be tricky. Opinion varies as to just how critical exact positioning of the post is, and some luthiers employ strange incantations and mystic rituals to get it just right. Others just shove it in until it sticke somewhere near the end of the bridge.

An additional problem with taking all the strings off together is that even though you get the bridge back at exactly the same place where it was on the fiddle, the bridge is tall enough that even a slight "slant" can change the effective lengths of the strings. Some say that it doesn't matter much since you "finger where it sounds right," but changing where you flop down the fingers even slightly can affect the "feel" of the instrument until you retune your fingertips (i.e. get used to the change).

An excessive slant of the bridge can affect sound quality, although "the equations" don't really support much effect if it "looks straight." But both fiddlers, and luthiers, tend to be mystics and have a variety of beliefs about just how critical "just right" needs to be. Their rationales were fully explained in an arcane book that was lost several hundred years ago, but the beliefs persist so of course they're true.

John