Perhaps I'm just overly cautious, but I try to avoid, whenever possible, changing the URL of any WordPress "thing" that may have a (hard-coded) link to it somewhere within WordPress. It is just so easy to have a "Global Search/Replace" of links go awry when trying to change the domain name of a web site.
The beauty of the subdirectory approach for all WordPress installs is that it makes a switch to a new install just so quick. A matter of seconds.
In my own case, I switched from the Twenty Ten to the Twenty Eleven theme with a redesign of the site in a different subdirectory. That is not practical for active blogs, but for traditional web pages, you can freeze changes for a week while you redesign.