The point is not to frighten. The point is to plan very well and have a good reason for doing it. Many times a well performing web site is deleted to replace it with some kind of flash monster.

With good planning:
mapping out all the inbound links,
content,
pages,
internal linking,
redirecting them with 301 header,
making sure the web server can support re-directs easily,
URL structure from old to new,

This is why you have to have a good reason for doing it and not because the new marketing person does not like the font. (CSS beside)

Now I'm thinking in the thousands of pages..  :P

If one had a few pages for their website then all this dire talk is not really relevant. Transitioning to some kind of CMS is a perfectly valid reason to 'upgrade' the site.

Probably the biggest reason for a redesign (outside of fonts= sarcasm) is incorporating a CMS system, or content management system. One might be going from a html based site (or the various flavors) to a template driven site with a database. Again many various flavors of this breed, WordPress being one of the Open Source versions. Joomla!, Mambo. HubSpot is one too. Not open source ;)
If you have a CMS in place then site redesigns that don't toss out the CMS are less traumatic then ones
that do. A lot of planning and testing is critical. Again that depends on scale too. If the site is a few years old then greater caution is required.
All that aside if you have a well performing site then redesign may well end up being a disaster. Window dressing stuff (colors, fonts, images) is not 'redesign' per say. Changing the URL and site page structure is.

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