What ashford means, is that they are very hard to replace. Cam bearings are made out of a soft metal, and are VERY easy to dents and ding. Not only does it require a speical tool (and a speical touch) to remove and install them, but if you do dent or ding one, then you risk causing some serious damage to the motor. How long have they been in there?
The books that I've read say that cam bearings almost never wear out. They certainly shouldn't be worn out after 50,000 miles.
If they've been damaged during camshaft removal/installation, take the engine to a pro for replacement.
edit: since you asked, for removal you need a special mandrel that fits the bearing ID, shoulders against the bearing shell and clears the bearing bore in the block. You'll also need a long bar to reach inside the block.
For installation: the bearings are pressed in. They must be pressed in squarely! Then they may need to be trued (there are various methods to do this, probably a boring bar is preferred). You can also blue and scrape the bearings. The books I've read say that it's a really bad idea to do this yourself.