You are upgrading your web site and as part of the upgrade, it means moving and renaming particular files.

Search engines have indexed your entire site and pages you're going to move or rename rank well. By altering these files, you run the risk of losing a lot of traffic and leaving visitors to your site who follow a search engine link with the dreaded "Error 404 - File not found".

A 301 redirect is the best way to go and I go into some detail on how to implement one in this tutorial, but first let's take a look at a couple of other strategies I often see mentioned around the web to get around the problem and why you shouldn't use them.

Custom Error Page

You could create a custom error page. The problem with this solution:
a) You will lose ranking for the page as the file will appear to be non-existent next time it's requested by the search engine spider. It could be some time before the page in it's new location or with a new name reappears and given you'll lose the power of inbound links from other sites to the page in question, it may not rank as well.
b) Your web site visitors may be frustrated by the fact that they then have to dig through your site to find the desired information.