This might help... it;s a little dated but the basics are the same...
Also note that you can sometimes force a reconnection if the "reconnect all" button is not active by holding down command.
Thanks for your help with this. That article seemed to be helpful, but I can't seem to implement it's suggestions.
The one secret to making manage referenced files work is that you need to select all of the files in the upper pane that are disconnected and then navigate to the correct/new location in the bottom pane. Doing this will activate the “reconnect all” button that will do all of the heavy lifting for you.
Unfortunately, when I try to do this, I am unable to "select all" -- I can only select one photo at a time. If I do cmd+A, all the files in the upper pane are not elected.. (And option + reconnect all doesn't un-grey that button).
Oh, man -- THANK YOU! That last bit of info did it for me.
So to summarize for others that may be struggling with this:
 Create a smart album to locate all the referenced photos (I used the keyword "iphoto original" which Aperture applied to all my iphoto pics when I starting using Aperture).
Select all photos in the album (cmd-A)
 File>Locate Referenced Files
 In the upper panel highlight one photo.
 Use spotlight to locate where that photo is on your hard drive.
 In the lower panel navigate to the location of the photo
 Click on the "reconnect all button at the bottom.
Aperture then took about 5 minutes reconnecting all my referenced photos.
Argelius and rwboyer-- Thanks for the clear step-by-step! This post just bailed me out big time. I'm pretty new to Aperture and just replaced my media drive with all the linked files. I saw the "reconnect all" option before but thought it would try to link all my pictures to one specific (wrong) photo's location.