jQuery change one of a number of class names for an item

jQuery change one of a number of class names for an item

I have a dynamically changing layout based on twitter bootstrap. It is a
grid, with grid items that look something like this:
<div id="grid">
<div class="row">
<div class="span12 red"/>
</div>
<div class="row">
<div class="span6 yellow"/>
<div class="span6 green"/>
</div>
</div>
Depending on user interaction, I need to be able to find items with a
certain span class (i.e. span6) - and change it to a different span name
(i.e. span4). How do I do this in jQuery without clearing out the class
names entirely?