[ricardo81]

Would say the most excusable part is the protocol but of course that generally ends up being a 301, albeit the URI has indeed changed.

Hostname, well perhaps if a company has been merged/sold.

Path/query is really down to information architecture and planning that early on can go a long way, e.g. contact, faq belonging in a /site subdirectory.

File extension doesn't really matter nowadays

Main thing is there's no technical reasons for the change. I recently saw someone wanting to change the URLs of their entire site because they now use PHP instead of ASP. They could use their webserver for PHP to deal with those pages and save the outside world a redirect and twice as many URLs to think about.