g1smd - 11:02 am on Dec 17, 2012 (gmt 0)
Canonical tags can't help here as the request doesn't result in a page of content being returned.
You'll need to set up redirects from the old to the new URLs. If it is a "simple" change in the URL, such as adding, removing or changing a folder name or hostname then one or two lines of code may be all you need.
If the URLs change in a non-simple way, e.g. example.com?id=543 becomes example.com/widget then you'll need to rewrite requests for old URLs to an internal PHP (or other) script that looks up the new URL in an array or in a database and then sends the correct HEADER 301 pointing to the new URL.